What do you mean "developed" at the same time?
Humans "developed" 2 million years ago, and modern humans 200,000 years ago. Human civilizations developed 10,000 years ago and modern tech levels developed less than 100 years ago. There are also falls such as the fall of Rome and the dark ages at the end of the copper age. There are civilizations that rise and fall due to others and some that do so due to their own stupidity.
My guess is "how do you explain several close species developing star travel and roughly equal levels of tech at the same time?" Well Let's say they all developed within the last 200,000 years and are all within a 50 light years radius. At some point in those 200,000 years they all get to the point of writing, farming, etc... What does that mean in terms of today's tech? Nothing. Is it surprising giving 200,000 years? nope.
Ok so some time after that point they reach the iron age.... How does that effect their technology as you'd see it? What exactly does that mean for history? Not much, because Greece, Rome, the Middle Ages, all exist within the same area of technology. Even science and the renaissance isn't going to matter much about when it happens. pre 1900s technology could go on for centuries, perhaps millennia and it wouldn't be surprising...
It is only the Bessemer Process that really makes things start moving forward to the point where multiple civilizations of the same type at the same level likely won't exist in the same time frame. Well I sorta lied. A civilization that has the Bessemer process just makes expanding easier due to lots and lots of high quality steel. This allows civilizations to build large and build crazy contraptions like Plains, Trains, and Automobiles... as well as the eventual LHC, but ultimately this just means that the civilization can get up to the point of a global to solar system based civilization, but the next stage after this, for all we know could take millions of years which makes similar civilizations more and more likely. Sure they'll be at different levels of converting their solar system into multiple colonies, but ultimately they'll be at pretty much the same level.
That's right, we're pretty much at a possible zenith that could stand for the rest of history, and there is nothing that says we'll decided to convert all matter in the solar system to habitats or just leave them as they are and live in harmony with nature more so. So you could have a species that has reached the same zenith who really have developed out the rest of their system or really young who have. They're both going to be roughly equivalent to each other at this point.
Now the question is how do they all get triggered to look out to interstellar space at the same time... Well, what if they are all older than "Earth" and all realized radio waves were useless long ago and while they have developed superior communication techniques that are similar to radio waves, because radio was such a waste of time each of their civilizations all gave up and never tried with the better stuff and they all just happened to have those time frames of people looking and emitting radio waves were just out of sync, but then "Earth" happens upon that superior communications tech and they're all roughly x distance away from "Earth" and so everyone in that area get the same "OMG SOMEONE ELSE IS OUT THERE?!" at the same time and thus begin working on interstellar technologies all at the same time.
Thus you have all the species both developed in the important areas at the same time, differences, and no "ancient alien"
Oh! and BTW, the ancient alien thing doesn't really work, because of what I described above. If 2 species start at the same point it doesn't mean they'll get to the same point at the same time. Rome could have survived without the burning of Alexandria and humans could have been in space 2000 years earlier. Or Persians could have won against the Greeks and Humans may never have even reached the moon due to the change of western philosophy.