The US likes to tie everything to employment - see also healthcare and retirement plans. Coming from Canada, anyone can open up an RRSP account (equivalent of 401k) and your employer isn't involved at all. You control the account. In the US you're stuck with the 401k account your employer signs up for. If they decide not to offer a 401k then you lose that benefit.
If you don't have access to a 401k at work you can contribute pre-tax to a traditional IRA instead. It is weird to have both, though.
Didn't healthcare get tied to employment due to union demands in the 60s or 70s?
The idea that international socialists have that domestic labor conditions are made worse by a lack of protections for foreign workers (rather than the presence of the workers themselves) doesn’t pan out if you look at Canada; it’s quite rare for status in the country to be tied to a particular employment agreement, so immigrants are free to leave undesirable employers, but wages are substantially lower there due to the country having accepted about 3 million immigrants and temporary workers in the span of the last ten years.