Because "Flyover Land" is still pretty expensive. There's a lot of reasons to choose H1-B. The biggest being that they need to be sponsored. That sponsorship puts the employee under the thumb of the employer. They can revoke that sponsorship at any time forcing the employee to leave the country if they cannot find a new sponsor.
Some corporate bean counter figures he can pay a college hire and a H1-B about the same. The college hire will gain experience and will want a raise. If they don't get it they'll leave. The H1-B visa holder doesn't have that flexibility.
The problem for the companies is that the current Visa caps means even H1-B workers are in short supply. This has not gone unnoticed to the folks that run the consulting companies that often sponsor the Visa candidates. I was talking to an IT manager a few months ago and they were complaining the Wipro wanted to raise their rates for Business Analysts to over $100/hr. Mind you the worker isn't going to see most of that money.
The current bill in the senate that has bi-partisan support entirely removes all H1-B visa caps. It's the nuclear option more or less for American Tech workers.