Masters nationals--depth of competition?

This may have been covered before; I'm new here, so I apologize in advance. I've been cycling rather than swimming for the past 10 years, and I recently got back in the pool as end-of-season cross training. Which of course got me curious about masters programs. I checked out the Masters Nationals psyche sheets and I was really surprised at the lack of depth of the competition. So heres the question: why do so few swimmers choose to compete after college? In comparison to cycling, or running, it seems there are hardly any swimmers competing at a high level over the age of 22 or so. For example, the mens 30-34 100 m free nationals psyche sheet shows 10 people, ranging from :56 to 1:03! 10 people? Is that because most people don't pre-register? Or are there really only a handful of 30-34 men out there who can go under a minute in the 100m free??