[Robin_Message]

That could be because a) anyone who can put together a coherent pitch in 30 minutes already knows the problem space and solution very well, and/or b) there isn't time to lie or exaggerate in 30 minutes, so if you fund them, they are actually as good as they said they were.

Any ideas which?

[pg]

Mostly a). The people who apply at the last minute don't usually have the idea at the last minute. They apply with something they've already started working on, or at least thinking about.

[networked]

Or c) a person applying at the last moment has enough confidence in his or her product that applying to YC seems optional rather than urgent.

That said, it would be strange if all the people who applied at the last moment, even all the successful YC alumni who did, did so for the same primary reason. I'd wager that it's more that there's a cluster of desirable founder traits that correlates with applying late, of which in any particular good founder a different one can dominate.

[sillysaurus2]

anyone who can put together a coherent pitch in 30 minutes already knows the problem space and solution very well

I'd guess most of the successful YC companies knew the problem space very well when they applied. That's probably a universal prerequisite to making something people want.