One thing that is somewhat surprising is how many people get into officiating and turn that into a high profile gig in the National Football League (NFL). Some of these cats just like football and volunteer to officiate a few local games -- high school, junior high and even lower. As they gain experience, you may see them rise in the ranks and eventually wind up in the NFL.
That is a surprising process to watch, but the reality is that even NFL officials generally treat it as a part time job rather than a career. Most people focus on a career rather than part time employment, and that means there's room available for others to get into pro officiating.