Porting is a mess. For starters, the list of employers willing to deal with your visa is limited (most startups are too small and moving too fast). Second, you can't quite your job, without having secured a visa transfer first. I mean of course you can but you must leave the country immediately (looking for job after quitting and before leaving the country is technically a violation of the law). While not 100% preventing moving between companies, it definitely puts higher pressure on job security.