Employment in California is “at will”, meaning that absent extenuating circumstances (such as a written contract) either the employee or the employer can terminate the relationship at any time, without cause. So, it’s not hard to terminate someone in California, but nonetheless employers are hesitant to terminate employees without cause, probably for fear of being sued.

I think it has to do with lawsuits being highly visible whereas underperforming employees are sort of an invisible cost. So, the risk assessment focuses on the rare but highly visible occurrence rather than the ongoing cost of continuing to employ someone who isn’t productive.

Just guessing. I don’t really understand this phenomenon.

Google can afford lawyers and PR to deal with an employment lawsuit but a lot of smaller companies don't feel like they can.

I had an experience once where a toxic, delusional, pathological liar of an employee was hired and started making up wild accusations about their coworkers on day 1. They then proceeded to do no work and had their role at the company progressively minimized so that they could have satisfactory performance for how little they were doing (and couldn't do much damage). There was no way the company was going to fire them and thankfully after a couple of years they decided to quit, despite making their entire team miserable that whole time.