That comes after the decision that you can't fix the situation, which comes after you discovered that the hire was bad, which comes after a number of visible failures. That's a lot of wasted time/effort, even if the firing itself is simple.

The cost of hiring, firing, rehiring approximates the position’s yearly salary.

Depends on the country I think - in Australia at least it seems like you can sue for unfair dismissal if you're angry about being kicked out, so HR departments only seem to get rid of someone as a last resort.

In my area they just tell you to leave. No warning. No severance. Midwest US.

The process is anything but simple at large companies. Even if the new hire is a complete fraud and can barely write code it'll still take an average manager 6-12 months to be able to show them the door. And it'll involve countless meetings and a mountain of paperwork, all taking away time from regular work. And then it'll take another 6 months to get a replacement and onboard them. That means your team has lost over a year of productivity over a single bad hire.