Couple important points:
1. SV/FAANGs, like elite colleges, only hire some very small percentage of people who apply. Many people who are perfectly well qualified and have successful careers otherwise get denied employment.
2. The market cap of these FAANGs is in the trillions. Between talent and capital, there's literally no problem they can't solve (they can solve problems about cyber bullying, trolling, etc but their business relies on it.. so it's not really a "problem" per se).
3. There are SO MANY underprivileged and/or poor kids in SV, LA, Pittsburgh, all the places where tech companies reside. Saying there aren't enough skilled workers in the US is kind of dishonestly true, as they have tons of applications from many qualified people.. many of whom with some training and support may very well become highly qualified....
4. ...They could literally invest some of those trillions to build a pipeline from these areas, develop talent locally - or at least domestically.
So much of tech is about solving problems with the resources you have. Opening up broadly to a global labor supply removes the constraint on that resource, and therefore don't need to innovate or find clever ways to make it work. Now, they are companies that have only one duty - deliver profit to shareholders - and that's fine and how companies should be run. But public policy needs to exist to serve the interest of the people, and for the most part - not entirely - H1B has been to serve the interest of executives in Big Tech.
Also, finally, Trump is absolutely awful, horrid, all that. A broken clock is right twice a day though, and this is one of the few possibly accidental things he's done that isn't a complete self-shot to the head.