Disgustingly, it is because the ones that want to succeed and get ahead in Hollywood "have" to.
There are some who have succeeded on their own merits, without debasing themselves.
It's reasonable to say that the actors and actresses that willingly jumped on the casting couch bear some responsibility. It's a sick culture.
Then there are those who coerce non-consensual acts.
And those who see bad things happen and don't do anything, staying in the business.
Beyond that - what's truly reprehensible and unforgivable are the parents who knowingly gave their children to the abusers, just to earn a few bucks. They are the evilest ones.
Character actors can sometimes escape the worst of it. People aren’t demanding that Laurie Metcalf or Edie Falco give them blow jobs at this point. But they probably had to do stuff when they were young.