In addition to what vkou said, firing individuals for low performance in the US, as opposed to mass layoffs, typically requires putting them through the PIP process (performance improvement plan), which is onerous and stressful for the manager. My limited understanding is it involves the manager, HR, and employee agreeing on a project and timeline that would constitute satisfactory performance, then constantly checking in and documenting progress. The purpose is for the company to collect proof the employee can't perform their job to indemnify themselves.

But I've seen people just temporarily work really hard and/or get lots of help while they're on a PIP, succeed at it, and then revert to old habits. In a lot of cases rather than go through the trouble to put a report through a PIP, which may not have the desired outcome anyway, the manager will just put them off in a corner and give them unimportant tasks, making it that much easier for them to just coast.