Well you just answered why they don't/won't do this.
The H1B employees being shackled and thus having little to no leverage to demand higher compensation or better working conditions is (from the point of view of the companies abusing the system) a feature of the system, not a bug.
The H1B employees being shackled and thus having little to no leverage to demand higher compensation or better working conditions is (from the point of view of the companies abusing the system) a feature of the system, not a bug.