Phillip Miller retired at 16-0, including 2-0 in the UFC. Why? There was very little money in the MMA back then. Even after winning his first 2 fights in the UFC, Zuffa low-balled him with $4k to show and $4k to win in a potential fight against Phil Baroni, who would have been receiving $40k for each.
Being a cop was a far more lucrative, stable career.
As for guys with only a few victories, many have no intentions of being full-time fighters, which is a very difficult, tough career, with severe uncertainty, as one might go long periods of times between fights, losses hurt your earning potential, etc.
And just because they're undefeated doesn't mean they're not getting their asses kicked by a guy/girl in their gym which indicates to them what their true level is.