Over the last months I've gotten very familiar with the tech job market.
Those job postings are ghost jobs. Companies usually keep a few generic jobs online continuously like "Fullstack developer" to fish for top candidates, but there is no actual position that needs to be filled.
Or because required by law for some reasons including visa holders (assuming that’s still a thing in the US these days - fortunate not to have to know)?
Yes its disgusting that companies rather hire cheap foreigners over qualified natives.