1. Horizontal farming and grazing cattle are cheap.
There's a school of thought that increasing technology leads to decreasing cost. That's true! But it's a generalization that leads to error. An improved tractor leads to more efficient farming (increasing the amount of food per-acre) — but the tractor consumes diesel fuel leading to pollution and while it allows for more robust farming, any modern farmer will tell you that the ruthless nature of economics is that it doesn't necessarily translate to an increase in wealth. Ignoring corporate farming, family farming is almost mandated to use the latest tech just to pay their bills due to the expectation of decreasing cost of food per mouth to feed.
But there's a limit to that. For example, some farms are part of a co-op and the co-op owns the combine used for harvest. It's trucked from farm to farm, because no one farm can spend that much money no matter how much more efficient the solution is.
Which is a long way to say that vertical farming (assuming we're trying to eke every ounce of geographic efficiency) requires construction, vertical water distribution, better management of sunlight (or provision of artificial light) and a lot of other expensive things. Why continue farming like your great-great-great-grandparents did? Because it's cheaper and there's nothing compelling you to invest in the latest tech.
2. Nothing else is demanding use of the land
Let's ignore environmental activism as we see it today. I think that effort is trying to figure out how to grow up just like the rest of humanity and so it's not an ideal comparison for what I'm about to say.
But if we assume (and believe me, I think this is a nearly unbelievable assumption) an ideal government run by calm, rational, conscientious people, then there will be an effort to preserve large swaths of geography for uses other than agriculture. Some will be parks of various varieties. Others nature preserves designed to protect natural ecosystems. Still others will be "open air" space between large, efficient, highly automated cities. In other words, in this wonderful utopian world there would be commonly-accepted pressure to preserve tons of open space.
Except that it's rare for people who read stories to relate to civilizations like that. We want those kinds of solutions, but we don't believe they can exist because they're nothing at all like the world we live in today or any aspect of our history. We've had brief blips that worked temporarily (e.g., The Farm), but deep down inside, we all know that we evolved in a competitive environment that will always be underscored by greed, ambition, and a host of other useful but occasionally detrimental behaviors.1
So why do people continue with traditional farming in your world? Because they can. Because there will be the homeless who squat on an acre to survive and families who can't afford the expensive solutions and governments who calculate national productivity in terms of acres in production and a host of other all-too-common social challenges that have always plagued us.
...Because technological development hasn't reduced land use yet and isn't expected to into the future.
3. Population control isn't as easy as you think
Oh, so many people on this Earth complain about the growing population and wonder what can be done about it. It shouldn't need to be said that the biological drive to produce offspring is whomping strong. We've tried everything from religious terror to national laws to guidance counseling to abortion — not necessarily under the banner of "population control," but that's the end result.2
But what has all these years of medical innovation, improved public health, bounteous food, and failed programs taught us? That the population will grow. It'll have its ups and downs, but it will always trend upwards. War can reduce population. Disease... affluence tends to reduce the desire for large families, too. But in the end, the number goes up.
And that means you need every scrap of arable land in production. Horizontal land still being farmed simply hasn't had the vertical farming structures installed yet. It'll happen. Think Asimov's planet Trantor, where every square inch of land (and a big chunk of ocean) has been covered over with buildings. It's just a matter of time, right? So until the government can get around to the correct subsidy to convert the Smith's farm to high-tech high-density agriculture, the cows still graze and the wheat's still planted "the old-fashioned way."
4. Forcing people to adopt technology doesn't always work
From a certain point of view, you haven't told us what "time" we're talking about. Are we early in the adoption of advanced agriculture? Somewhere in the middle? Late in the adoption phase where pretty much every acre that can be converted has been?
My workhorse vehicle is a 1992 Ford F-150. It's had the snot beaten out of it. I've dropped a tree on it (a painful story about classical physics). The door handles have been replaced with rebar because they kept breaking. My point? That 30-year-old vehicle still runs great even though it's nowhere near as fuel efficient or pollution-reducing as modern vehicles — so why spend the money to upgrade?
Back in 2013 the U.S. government decided to help the nation adopt LED technology by outlawing the manufacture and sale of 60-watt, 75-watt and 100-watt A19 incandescent light bulbs. Suddenly senior citizens on fixed incomes were forced to pay \$25 for a light bulb that once cost \$0.50.3 The argument that it would last for 20 years falls flat when you're talking to an 80-year-old person.
The second round of efforts to force the nation to use more efficient lighting occurred this year (2023). Ten years passed before step #2 for many reasons, but one of them is that it took time for the nation to grow accustomed to what was perceived as one whale of a bad idea.
My point with #4 is this: while the cost of LED lighting has gone down, it's still a long way away from $0.50-per-bulb and likely will never attain that value. A (hopefully unforeseen) consequence of forcing the adoption of new technology has been inflation. We might be saving in energy costs in the long run, but we're paying a higher price for the bulbs. In 2013 LED bulbs were advertised as lasting 25+ years. Today the average is 10-13 years. The economy is trying to make them as consumable (i.e. "throw away") as the old incandescent bulbs.
Consequences like that stop people from simply buying electric cars or putting solar panels and wind turbines on or near their homes and businesses. It stops people from buying the latest-and-greatest cell phones and computers (most people don't...).
And it'll stop farmers from simply paying the cost of installing expensive farm equipment. You'll have traditional farming because some people simply don't like change. Why pay for all the fancy doodads when what you're doing pays the bills just fine?
5. Is every nation on your world technologically and economically equal?
Finally, it's unlikely that your entire planet is homogeneous. Some countries or areas will be wealthier than others. Wealthy countries/areas can provide subsidies to help convert farms to the advanced techniques. Poorer countries/areas can't. It's not always possible for the wealthy areas to pay for the poor areas.4 Using Earth today as an example, there will be sizable chunks of land farmed using traditional techniques simply because those farms are in an area that can't afford the upgrade.
You'd be surprised how many worldbuilding questions depend on economics....
1 People who think it's a good idea to rid humanity of, e.g., greed haven't through through the implications of that. Those strong characteristics that can lead to very real tragedy are the root characteristics that also lead to innovation, problem-solving, safety, and a lot of other beneficial consequences. It's nice to look back from our modern perch and proclaim that we could have done it another way — but the nasty truth is, we didn't and couldn't. Without fundamentally changing what it is to be human, our goal is to figure out how to train ourselves to not give in to the darker side of these characteristics. People don't relate to utopias because we've not shown a lot of progress in this regard.
2 I'm really not trying to start an argument here. Specifically citing abortion, the issue is quite a bit more complex than simply life vs. women's rights and I believe the issue has been politicized by a lot of people and organizations — including those seeking population control.
3 I'm simplifying for brevity and it's not completely fair. Halogen-based A19 bulbs were, at that time, still permissible so the price jump was really from \$0.50 to \$4.95. But I was there when those first people had to have the government's decision explained to them. The reactions reflected the \$0.50 to \$25 condition.
4 If you don't believe this, look at how your nation, state, or province deals with "education equalization." This is your regional government's effort to take taxes in the wealthy portions of the region and use them to standardize public education throughout the region, including in the poorer areas. It's a desirable idea to ensure that everyone has access to the same quality education! It's also whomping difficult. It's probably true that 99% of the world's wealth is held by 1% of the world's population. What people don't get is that taking all that wealth from the 1% and spreading it out among the 99% doesn't significantly lift the 99%. And the challenges faced by "education equalization" goes a long way to prove it. Soapbox mode off.