I suspect for most people, it's because a full marathon is at the very limit of their abilities, and a rare event. Most runners will run exactly one; even runners who keep at it will run only one per year. They'll need course support (including the enthusiasm of onlookers and other runners) just to be able to complete it. And they'll want to celebrate it and commemorate it with all the rigamarole that accompanies an organized race: the shirt, the medal, the banana.

None of that's necessary, of course, but if you think of it as "150 dollars per year" invested in the sport, rather than "a really expensive day doing the same thing you have been doing", the fees don't seem like all that much.

For me, those fees are motivation: I don't want to fail or look stupid at something I paid to do. Stupid, I know, but it's kind of a dumb event anyway. I'd be in overall better shape if I didn't run 26 miles in October and then take two weeks off to recover.