That's not at all true. People on H1B cannot change jobs easily. They have to go through the visa transfer process and hope nothing goes wrong with the process. They have to make sure their current employer does not get wind of the job change and doesn't fire them during the transfer process. They have to ensure that the new employer can process their green card application. Lots of nuances involved. It's a serious source of stress for a lot of highly skilled H1B engineers