Because unless you have a big name/brand already your best shot of climbing the ladder is in the UFC. No other org has this kind of reach or following or big fights.
UFC is by far and away the top MMA organisation and most of these fighters want to be there.
This gives them a lot of leverage and yes they do take advantage of that leverage. But most of these fighters want to be here and also make more money.
Look at masvidal, who would he fight if he leaves UFC? Sure as hell isn't getting the conor or nate fight. He could have 1 maybe 2 money fights in boxing where he'l get his ass handed to him. Or some mid level fights in another org which he might win but won't pay as much and won't raise his stock too much.
That's the reality of it. Until there's a proper competitor at UFC's level it's the best place to be.