Sherrifs are complicated. First it is important to understand that police are tied to the cities they serve in. There are parts of cities are are not incorporated and thus need a different sort of law enforcement. Mostly remote places. But it does happen in denser areas.

Sherrifs are elected law enforcement. You can "run" for sherrif and just be some rich guy looking to play cops and robbers. See Texas.

However there are sherrifs officers that are on the same wavelength of police and just to enforce where the lines keep the police out. And you might think "Well why don't they just incorporate the parts into the city?". Lots of other issues with how the land is distributed in other ways.