So open walk-ins could account for a lot of lower ranks. And most of the people in Starfleet seem to love it, regardless of their position. No one seems to be hoping for the day next week when his committment expires. Except guest characters in a war zone who get killed in the last act.
And Federation public schools probably have Starfleet recruiting commercials running all day in the halls. "Sure, Federation life is good! But with Starfleet, life can be better!"