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.

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.

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.

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

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