That's the thing, though: they don't, usually. Falsetto doesn't simply mean forcing your voice to be higher than it is naturally (which is, of course, very common in anime), it's a particular way of using your voice. You can also create a higher pitch with a head voice or even a chest voice. Falsetto is the easiest way to do it, but it's also the hardest on your voice, which makes it hard to listen to for prolonged periods of time.

It's not just the squeaky little girls either, even the young men are always very throaty.