Actually, depending on what sorta spawn it is, then 1 exit could work. Look at frontier, RED's forward spawn has only 1 exit, but near the point, basically inviting in spawncamping. Similarly, look at Badwater's shortcut that closes later on- it can basically count as a spawn exit, since it requires the enemy to push through it in order to seal off a heavy flow of REDs
That other maps do it doesnt mean that its allways good. Yes, it could be a minimal flaw because the situation is extremely rare. And in the case of frontier, if it was a real issue then during a test it would already have shown to happen. And even then, depending on the way the exit is built it might not be decent. Thunder mountain shows the perfect examples for explaining why and what is bad, and what is good.
In stage 1 there are 2 exits, both directed into the same general area. This allows all spam to focus on just that single point. A bad team vs a good team will heavily stack to the good team because they have a 1024 wide path with alot of cover to spam into 512hu wide path (which has 2 doors of about 192 wide). Normaly for a spawndoor the flow of enemies coming out of them shouldnt harm. But on realy bad teams they wont be able to overtake a strong defense. They would be locked in and face the ultimate game of boring. Facing that, nearly everyone would rate the map a 0/10. Its not fun.
Stage 2 shows the same 2 spawn door system. except they are directed away. this means they can still focus fire on spawn. but need double the players to essentialy lock it down. Adding to that, since they reached stage 2 they are often strong enough to do so. See the same on gravelpit where you have 2 of the dropdown areas per cp. It forces them to prioritize one exit. And in a 12 vs 6 situation when trying to push out it becomes alot easier. They might still be quickly stopped at a point directly after that, but they have room to navigate and attempt a more decent attack. This is why this already works well. But in the end, its still not ideal when its placed on a first stage. Again, the worst teams might still get locked down because 1 sentry can overlook both exits (meaning 2 demoman can do the same).
For stage 3 there is a gap of over 1024 HU between the 2 exits, and there is a 3rd one which again forces a detour. Have you ever seen spawnkilling in a way they cant leave spawn happen there? Probably not, because if you did you spotted a team that probably has problems even pressing m1 or goes 12 snipers. These exit system sare the safest possible and very reliable. And they arent even forced to be connected, although it is more recommended.
To compare with valve/community maps. Look how many of them have a similar spawn door system in its first (or its only) stage:
Dustbowl, gravelpit, goldrush, 2fort, badwater, badlands, pipeline, hightower, snowplow, powerhouse etc.
All these maps have multiple exits in its first stage and often enough also on the 2nd or 3rd stage. Only dustbowl and goldrush dont (and guess which rounds on those maps are problematic)
Stating 1 exit is enough is only justified because you havent witnessed the worst possible teams. These exits might be locked down because they are bad, but that even you cant take some action other than going medic to increase the chance to leave is bad. If you can make more than 1 exit, always do so.