And if that employee can show that other people were allowed to wear green shirts without being fired, they can use that to support an illegal discrimination claim against the company.

The reason it takes effort and documentation to fire people is because companies want to have a uniform set of rules that are applied equally to all employees. They also want documentation to support the firing. Having a consistent process, applied equally and with supporting documentation discourages people from even trying to bring frivolous lawsuits.

Having additional process is also a check on managers. Some managers try to fire anyone they disagree with, dislike, or even to do things like open headcount so they can fill it with a nepotism hire. Putting process in place and requiring documentation discourages managers from firing people frivolously and adds another level of checks and balances to discourage gaming the system.