Nobody wants to ruin the party. You're asking someone to stand up and say "stop making so much money, this isn't sustainable" when EVERYONE is making money hand over fist. Sure, it may be the right thing to do depending on your point of view, but nobody will do it because they will be laughed out of the room and cast aside.

Retail share holders are ignorant, so they don't matter. Big share holders know that there is a time frame that they care about that is as short as they want it to be. They don't care about the long term. The board of directors (in theory) should, but even they are short-timers. Asking them to stop making money in the short term to help the long term is counter to their personal interests. It's just a flaw in the system, and a major argument against going public.

Interestingly, the handful of folks I know who have served as directors for public companies all share certain personality trais: A strong extroversion (in that they seem to care a lot about what others think of them), a tendency towards overconfidence, and an aloof demeanor. Not sure if that's universal, but it's something I've noticed. It probably doesn't help.