It's not practical. I looked into it once on a related matter and unless you're a US state dept employee or member of the military, the fees and taxes on importing and registering a car bought outside the US are astronomically high, like the cost of the car all over again, tens of thousands of dollars. The US has very specific and nit-picky safety standards. It's not a matter of meet or exceed, it's a matter of meet and don't exceed the limits imposed by law or regulation so you can't buy and bring over a European car that has better safety features if they don't meet US standards, so in that case you have to pay a specialty mechanic to adapt your car to US standards. And even if the standards are met exactly there are huge taxes and fees on it to discourage people importing cheaper cars. This is not the first US rodeo with protectionism for US car companies, they did this whole thing with Japan in the 90s and the Japanese eventually just did bulk importing and then later set up US plants for making the cars to get even lower costs.

Those for whom it might be practical would represent a few hundred thousand, maybe a million or two people who live very close to the Mexico border and don't just not registering in the US and dealing with the Mexican paperwork regularly.