Maybe a little hyperbolic, but the point is that it adds an additional significant shackle.

If I decide my company treats me like crap, I can apply to any* job in the US that I'm qualified for.

If an H1B feels the same way, they need to search within a smaller subset of jobs that are willing to do that paperwork. Last time I was hiring (2025) we were not open to sponsoring H1Bs or green cards. It's just a lot of extra work and legal expense, and frankly we found plenty of talented developers so we were right to not do it. Tons of employers don't deal with it.

* Note: Adjust this figure down somewhat to reflect the fake job postings that I never had a shot at, because the employer is just pretending to look for domestic candidates to satisfy H1B or PERM requirements.