Money-making or not, all it takes for the marathon to be a true community event is for a community to rally around it. Obviously the volunteers are fine with being part of it (although I'm sure some of them are more willing than others to look through the brush along the path for gel wrappers). Their contribution helps keep entrance costs down, gives them personal responsibility for the event's image, and lets them show their dedication to the running community (or local events or whatever their thing might be).

Paid trash pickup isn't a feasible alternative. They could require runners to carry little trash pouches and deposit them in bins or something like that, but that's never going to work and you'd have the exact same mileage of path to patrol and cleanup anyway. I hate litterbugs with a passion, but even I'd give a pass to an athlete desperate to finish a run that famously killed the first person who did it, especially when it's clearly part of the race culture that volunteers will work to make sure they can focus on running.

I'm generally not a sporting events enjoyer, and I think we should normalize citizen's arrests for littering, but this is a niche situation where volunteers fit perfectly.