This also forces companies to filter out "possibly good" candidates and only hire "probably good" candidates.
If it were normal to get fired after a day or a week, you could get hired at 10 different companies over a span of two months and likely find a really great position, where you're a great fit.
My next company will have explicit rubrics for what it takes to get fired, and your status will be tracked daily. You'll know at all times exactly how close you are to getting fired with how much severance. It will never be a surprise, unless it's a reaction to an acute event (sexual harassment, etc). We'll hire pretty much everyone who walks through the door with a plausible story for how they add value. Anyone we fire we'll sit down in the exit interview and write a plan for how they can get re-hired in a few weeks or months if they're interested. We'll also try to spin off a separate business with the fired person at the head, instead of just firing, whenever possible.