This graph shows that 55% of people have found less than 100 caches, and 85% have found less than 500.
In my experience when people start geocaching they are often very excited and rush around finding the caches near where they live. When they have found all the nearby caches, they have to drive a bit further and expend a bit more effort. This is when a lot of people stop, or slow down either through time pressure or lack of interest.
The rest are a few diehards who have usually become addicts. More on this at another time.