Let's look at this from the opposite point of view and see if it still works
I'm a nation looking greedily at yours. You have resources I want. Oh, not your cities. Who cares about cities? All those people to feed and services to provide. If you're willing to hunker down in your cities to basically wait out the process I'll send you flowers.
No, what I want are resources. I want oil, coal, lumber, some of that gold from your central mine would be nice, too. And since I'm not stupid and I know you're not training a regular army and a few spies have whispered in my ear that you're perfecting partisan warfare, I've concluded you can't be trusted to fight me face-to-face, mono-y-mono, man-to-man. You're actually willing to put your entire population at risk in the hope that you can harass me out of your country.
Huh... Good luck with that. I've been reading through Terran history books and there wasn't a single war actually won by partisans or guerillas. Oh, they contributed to the fight. But the wars ultimately could have been won without them. They're not indispensable because, of necessity, they can only fight or act in relatively small groups, hiding in the shadows.
Now I know what you're thinking! Vietnam! Korea! Jungle warfare, where every step you take is hidden! Except that you don't live in that kind of area. In fact, your sizable and reasonably same-tech-level-as-my-country people depend on large swaths of irrigated land, hydroelectric dams, transportation like boats, trucks, and trains — all those niceties that you'll try to use against me. But unlike that Terran gentleman Sherman, who marched through Georgia leaving railroads, farms, and housing completely devastated (his signature bent rails were known as Sherman's Bowties) and who had an entire army at his beck and call, you have small, well-trained groups of people who can't actually blow things up faster than I can rebuild them.
What, the American Revolution? Well, yes, it could be said it was won through guerilla warfare. It wasn't. Their standing army grew over the course of the war and, in the end, it was that army that won, not small guerilla groups. And it was in a time when it was incredibly difficult to land great numbers of men to fight them. You don't live in that era and I can put a half-million-man army on your soil comparatively quickly. And since you're pretty much giving me your farms, I can feed them.
What, Afghanistan? You're really not looking at this from my point of view. I want your resources. So what if the Afghani people drove the Russians and then the Americans out of their country after, what, years? decades? By that time I've stripped you clean of what I wanted and you'll be rebuilding far longer than I was destroying. You might as well declare yourself a 2nd-world or a 3rd-world country right now.
And I haven't even gone into detail about using missiles (your partisans are worthless against them) or advancing with whole divisions of artillery, or carpet-bombing from the air. I was really interested to read about all that lovely gas used during Earth's World War I. If you're not going to fight fair, why should I? I'll just send clouds of all kinds of stuff down wind into your cities. Ooooh. Never gas. That would decimate your cities (and I never even entered them!).
So, I march my armies into your country. I don't take your cities — you've already killed yourselves by deciding nothing is worth defending before I occupy it. Your larger cities depend on piped water, so I'll cut those off. I'll stop the trucks and the trains and the boats and sit down and wait. I don't need a large force to contain each city because you don't have a large force to throw at me.
So, while you're hemmed into your cities, starving (it just takes time, no matter your stockpiles), I'm in control of your farms, fisheries, mines, timber, basically everything worth having. And the only price I have to pay for it is somewhat higher than normal maintenance costs and the occasional loss of soldiers' lives. Oh well, people die, it's what they do.
And after a while, when enough of your people have died from starvation, they'll beg me to set them free and give me pretty much anything I want.
Nah. You can keep your partisan and guerilla warfare. It's penny-ante stuff. They're useful for a resistance, but a resistance is basically just annoying the occupying force until the big boys bring in the big guns. Yeah. I'll keep my armies and navies and air force and missile silos.... They may cost more than your forces to train and maintain, but they're tried and true.
The fundamental problem with partisans and guerillas is that they're an after-the-fact solution that assumes the attacking army is embracing morals. So long as the attacking army is unwilling to attack civilians, partisans and guerillas work. The moment the attacking forces no longer care about civilians, partisans and guerillas are worthless. Remember, carpet bombing... missiles... artillery... tanks... Even if the partisans and guerillas succeeded in pushing the attackers out, so much of the country would be lost that the next attacker with fresh troops and supply lines can simply walk in.
And don't forget that your country is constantly changing. Babies are born, cities grow, industries expand. Your partisans and guerillas are useless for helping the economy grow because they'd be constantly training for every change that occurs. Every new bridge. Every new house. Every new acre of farmland. There's always more and more to defend and the plans, storage, etc., would be constantly changing and updating to keep up. Ugh.