This seems like a failure of YC - if companies that have gone through the program are consistently forgetting the lessons it has taught as soon as they leave, maybe they're not being taught correctly?
That said, I'm not surprised. YC is a rare opportunity, and people will absolutely punish themselves for the duration of their stay in YC to get the most out of it. Few people can maintain that kind of pressure longer term. I don't think that's a weakness, I think it's just being human.
Of course it's a failure of YC. There is a 0% chance YC has all this stuff right. It would be depressing to them if they did: that would mean there were no further opportunities to improve the outcomes for YC companies.
I think it comes down to discipline. Not doing what you know you need/should do is a discipline issue. There is probably a lot of incentive to do what needs to be done during YC when everyone is watching, and preparing for demo day.
That said, I'm not surprised. YC is a rare opportunity, and people will absolutely punish themselves for the duration of their stay in YC to get the most out of it. Few people can maintain that kind of pressure longer term. I don't think that's a weakness, I think it's just being human.