Probably because immigration reform almost never means just throwing down the borders and reverting to the old rules where you only had to prove a few years of residency to be eligible for citizenship.

Partially because racism, and partially because even a decent number of our allies would take extreme umbridge with the US doing that because of how it’d brain drain basically everything where the US offers a more competitive standard of living for given income.

The European tech sector would more or less instantly vanish because of how much more well paid American coders are by comparison, and Mexico would find itself losing every scrap of development and progress it has achieved over the last century as every educated professional files straight across the border.

Would do wonders screwing over America’s adversaries though considering how literally all of them have such a worse standard of living that they have to set up structures to keep their people in as opposed to keeping the rest of the world out.

Basically the platonic ideal is to brain drain our adversaries, and strike a state by state balance with everyone else for what sort of freedom of movement arrangement most benefits both countries, the Philippines for example would be over the moon for a more open immigration system with the US since brain drain impacts them a lot less given how prevalent remittance is among Philippines expats.

Vietnam likewise would probably negotiate something more akin to a military exchange program since the war has become water under the bridge when compared to the ever present threat China presents to them, and Americans are some of the only foreigners the Japanese are able to tolerate enough to not actively scare off expats and immigrants. Brazil would LOVE an incentivization for American rocket scientists to brush up on their Portuguese, the idea of becoming a space port super power makes Brazilian state leaders drool over themselves at the possibilities, especially if French Guyana stops cooperating with the ESA.