Because America has a moderately[0]-restrictive immigration system, people who come here on a visa are committed workers. If you decide you don't want to work for one company after moving here, your options are to leave the country in 30 days or find another firm knowledgeable enough about the immigration system to adopt your existing visa application.

The reason why this happens is because the system is designed to be deliberately difficult to use. The idea being that if it's hard to hire an H1B visa worker, you'll hire an American instead. But because this works as intended, companies that know how to work the system get access to hordes of cheap, pliant labor. So instead of protecting American workers, we protect well-connected businesses that can screw over American workers.

[0] Compared to EU nations, at least