As someone who has gone through the immigration process, I can affirm that once you are no longer beholden to your employer for work authorization, more opportunities are available, and that leads to significantly larger salaries.

I am sure a large part of it has to do with the fact that until certain points in the process, you cannot change your employer without restarting the process, so a lot of people just stay without making a fuss about it.

Edit. Regardless, as it stands, the current H1B is indeed a broken system ripe for exploitation.