Why do some intelligent people lose all interest in academia?

Money.

Culture.

While there is certainly an "academic ideal culture" that people imagine. Being able to research interesting things, share your knowledge, take sabbaticals, etc.

It is a grueling path to get there, and when you do get there you realize that because of the mostly small close-knit departments, everything becomes extremely political.

This happens in private business, but in business your avenues to power and influence are usually a myriad greater than in academia.

Struggles within your department feel like family fights, not work just bullshit.

The overwhelming progressive, leftist bias in academia is attractive to some. If it isn't you thing you really have to be well prepared for cancel culture.

I once had a professor tell me she had to register as an independent because you couldn't be a registered Republican in her department. And this was 20 years ago. From all accounts it is 10x worse today.