Why is difficult to fire employees

Question from an individual contributor to managers at tech companies or tech teams: why is it difficult to fire under performing employees on the team? I’ve experienced several occasions where a co-worker was clearly underperforming (missing deadlines, not completing tasks, ignoring communications, and even falling asleep at desks) but they would never get fired for at least 6-9 months. In some cases, the manager opts for managing that employee out or forcing them to quit instead of outright firing that employee.

It makes it really difficult for the rest of the team when it’s obvious to everyone what is going on. I had to play along and pretend like the task being assigned was gonna get done even though everyone knew it wasn’t going to happen and knew that the task would eventually get reassigned to me or another team. It was frustrating, demoralizing, and demotivating the entire time.

Not being a manager myself, is it more complex and political than just documenting all the mistakes and tell HR that person is getting fired?

Edit: Since someone accused me of karma farming…I’m interested from a management perspective. I’m very much aware there are people and livelihood involved that have varying circumstances and issues. I’m specifically asking the difficulties of letting someone go who is clearly not doing the job they are paid to do and deserved to be fired for various reasons.