They don't have to undercut their coworkers on an individual level. When a position that could otherwise reasonably be filled by an American is gated for any unnecessary reason, be it undesirable pay or excessively specific requirements or whatever else, that effectively removes that position from the domestic job market. When that is repeated many times the end result is the same number of domestic applicants competing for fewer positions. That results in downward pressure due to basic supply and demand.

That's fine if there's a genuine need for a specific sort of specialist and the US simply isn't producing enough of them. But when it's silly hyper specific requirements it becomes detrimental.