Many municipalities are loath to do this. It's not easy or cheap to replace cops. Good ones are tough to find and not cheap. Sadly, the profession attracts the type of person that absolutely should not be doing it. Sometimes the pickings are so poor, that departments will get desperate and hire marginal candidates. They often have to resort to trying to steal good cops from other cities. This isn't cheap.
I learned about all this when I lived in Oakland. It's wild west there with a shortage of sheriffs. So they would put special bond measures on the ballot and the citizens would approve. They would advertize great salaries and benefits and then hold expensive training academies. Hundreds of people would show up and in the end around 95% would wash out. Many due to psyche evaluations. To hire 70 new cops takes academy after academy.
Then there is retention. In the case of Oakland, the beat cops working there knew that after only a couple of years in Oakland, they could write their own ticket anywhere else in California. I guess there is value in battle hardened vets.
It's a tough job being a cop. It's easy for me to see how after coming face to face with the scum of the earth on a daily basis, getting little respect from the community, witnessing revolving door justice first hand and realizing that you have real power over people, that some cops would go rouge. Maybe we really do need robocops.
Anyhow, the point I'm trying to make is, there are reasons a town, or a department wouldn't want to just sack the whole team and start over. I'm not saying it shouldn't be done, just that it is tough.