Not sure if it applies to H-1B but if a company does mass layoffs, it automatically makes it so that the PERM applications (required for green card, which you need to keep the employee past the visa validity period + extensions; up to 7 years iirc) will be automatically rejected for some time. So it screws over your existing H-1B holders, making your company way less attractive.

Source: I came to the US on H-1B in 2012. I may be misremembering which stage of the process the mass layoffs affect.

Part of the problem is you don't know ahead of time (certainly not with 100% certainty) who's going to be an exceptional unicorn wrangler, and who's just going to be a pretty good engineer, unless they already have an incredible track record elsewhere. This will filter out a lot of possible future unicorn wranglers.

Geez, maybe there shouldn’t be tens of thousands of hires over Americans then?

Sorry, it’s just that maybe a LOT of you aren’t understanding the motivation here?