Hiring people is very expensive. Finding and qualifying leads, creating job description, interviewing, on boarding, orientation, training, introduction to and getting familiar with the existing code base. Firing requires distributing that team member's work to other team members.

This can take months and a big investment from existing team members, to go from wanting to hire someone to a fully productive team member. And that's assuming your hiring process never accidentally hires someone even more incompetent than your "poor" team members.

Compare that to investing in improving existing team members to help them improve. Is that really more expensive than hiring and firing?