Sure, but turnover has a strong effect on employee morale and the company’s reputation

So even if it surfaced the best candidates, hiring anyone that applied and then firing 90% of them after a week would result in a depressing work environment and an ever shrinking pool of qualified candidates (who wants to upend their life to take a job they only have a 10% chance of keeping?). Not to mention the cost of onboarding.

This is how we get to the status quo: do as much filtering as you can before you hire.

Just because you are legally allowed to do it, doesn't mean you should do it at the drop of a hat.

Slacking off during the hiring process with the intention of firing a large portion of the people you hire is a violation of the social contract and a huge asshole move.

This is true. Workers in the US have effectively zero rights to employment until they’ve been employed for at least a year, and even then you can still fire them and just make up a reason. Corporations spend a lot of lobbying dollars to make sure the US remains a corporate authoritarian state.