Because some of their employees are in one or more protected classes and if you don't have a legit reason documented it's much harder to defend against a claim

Because some of their employees are in one or more protected classes and if you don't have a legit reason documented it's much harder to defend against a claim of illegal discrimination. And you can't just gather those reasons for employees in protected classes or that itself is discriminatory treatment of employees. So you have to make a choice as a large company: PIP systems or set aside more money for legal fees and settlements. Most opt for the former.