Players who sign with Boras know exactly what they're getting. There's a reason they sign with him. His track record of success outweighs the obvious conflict of interests, not to mention players hilariously don't care about that.
I don't know all the history of every player but I imagine a lot sign with him out of college because he can maximize draft earnings and then stick with him because it's easier and he's really good at navigating the arbitration process.
Additionally his company also is more than just him at this point.