The US is insanely litigious and most large companies are very aware of the cost of unlawful termination litigation. That means to actually fire somebody you need cause, and that means having an airtight paper trail which takes time to generate OR having the employee do something that directly violates something you've explicitly told them in writing they can be terminated for. You're not generally dealing with stupid people, and if they're obeying the letter of the law/policy even while defiling the spirit of it you really can't do much to terminate them.

There are other things you can do to make it clear they need to move on but it's usually way easier and more efficient to just directly negotiate a severance and have them resign.

I didn't say that. It's a complicated situation, right? If an employee threatens to kill another, they can be terminated immediately (with security escort).

i'm talking about employees who are in good standing, like Timnit was at the time. She had just been previously promoted. A person like this, even if they are annoying many coworkers through emails or papers, can't just be terminated because the manager doesn't like them (huge liability risk). Instead, Google (or IBM, or whomever) wants a paper record showing that somebody is unable to do their job, is put on a performance improvement program, cannot improve performance. At that point, Google can terminate the employee and if there is a lawsuit or mediation, Google has the paperwork required.

I think there are complexities with firing some people... If a manager just walks round firing people with little evidence, all other employees will live in fear of being fired. That isn't good for morale, productivity or creativity.

Instead, the problems of the to-be-fired employee needs to be abundantly clear to everyone nearby, so that when that person is fired, it doesn't have deep social impacts.

Combine that with desiring to fire someone when a replacement is trained up (often many months), and at the end of a big project (sometimes a year), and not just as management is reshuffling... And suddenly a firing takes 1.5 years.

A friend of mine, a non-technical manager at Google, similarly struggled to fire an obvious underperformer for over a year, so I totally believe it.