Dentist in St. Louis is two searches, and the county line splits them
The head term is ambiguous here in a way it is not in most metros, and the ambiguity is expensive to ignore.
Someone typing dentist in St. Louis might be standing on a corner in Tower Grove or sitting in an office park in Chesterfield. Those are different governments, different drives and, for your purposes, different markets. Locals say the county line and mean it.
Search engines resolve that ambiguity with the searcher's location, not with your title tag. So the broad query returns a rotating mix of directories, group practices and whichever office is closest to the phone. Beating that outright is a directory fight, and it is not your fight.
The alternative is to rank for what patients actually say out loud. Kirkwood. Webster Groves. Florissant. The Hill. Those phrases carry less traffic each and far more intent, and the competition for them is often a single competitor with a thin page.
Getting this wrong has a specific cost. A homepage stuffed with the metro name reads like an out-of-town chain to a local, and it ranks for nothing in particular.