They're Not In Charge of Cops
Cops and FBI agents are two totally different entities. If anything, they have to work together on a case. The FBI won't "take over" like they do in TV shows and movies. The public perception of the relationship between the cops and FBI agents is purely based on entertainment media. The FBI will never run into a case and "kick out" the cops!
The FBI Director Has Term Limits
Even the director of the FBI has a term limit! The original FBI director was J. Edgar Hoover. He was in charge of it all for 48 years. After he retired, congress decided to have the position limited to just a 10-year term. Therefore, there is only one director per decade, which seems to be working out. After all, it's very much a job as much as it is a position of immense power. Term limits are important!