Because the supply of qualified applicants for ANY job is unimaginably larger than anyone imagines. Even if you're hiring assistants for the "best researchers in the world" -- which, let's be honest, are all doing the "10 years and retire" game at FAIR/Google/etc. anyways these days -- there are just unimaginably many qualified candidates.

Even killing the H1B program entirely wouldn't directly effect the ability of universities to recruit research/teaching assistants.

Consider if any US trucking line could hire literally anyone in the world. Wages would be below subsistence level. Which, well, lots of the Research Assistants working as Software Engineers for National Science Foundation portfolio companies are barely making enough to support themselves.

This doesn’t jive with the post I was responding to.

Because these wages are near universally low, so no one needs to compete on that. Also for the same reason they're recruiting international students in the first place: Money.

(Note: this doesn't mean the schools aren't fulfilling their educational missions, it just means that revenue is both necessary in itself and as such a core metric that is pursued.)