St. last names?

Question

why is it some people have names that start with St. like St. Vincent or St. Dennis?

Answer 1

Often those are French names -- long, long ago, people who adopted those names did so either because they were from towns with those names, or, they wanted to emulate characteristics of those particular saints.

Most, though, were from the towns of those names.

Answer 2

Good theory! Sinclair, for example, comes from St. Clair. The Scottish Sinclairs were originally Norman French with origins in Saint-Clair-sur Epte.