Run a race, you’d be surprised. It’s a very communal activity.
Remind me to decline the invitation for your home cooking. :o
[ul]
[li]You get a shirt / other swag. [/li][li]The ability to run non-stop, even if that means just crossing the street w/o needing to wait for the traffic light. (Don’t forget to wave at Boyo Jim. ;))[/li][li]The good feeling that you’re helping someone/a charity out. If you’re going to run anyway, why not have someone benefit from it.[/li][/ul]
I get those…I am more puzzled as to the reasons why people sponsor running. Although running itself has zero appeal for me, so I guess that’s probably one reason why 
Because it’s easier to rationalize than just giving someone money for a cause.
I ran the Boston Marathon and raised funds for a local organization to get my bib. I never would have qualified for Boston otherwise. I never would have raised money for that cause otherwise. I never would have asked my friends and family for nearly $3000 otherwise. I may never have run a marathon otherwise.
It’s a focal point, it’s a way to start the conversation, it’s a way of putting skin in the game, it’s a generally accepted social norm.
Golf scrambles for charity are pretty common around here. A day off from work (hey, it’s for charity! and golf and drinking.
Probably most people who run in charity races don’t actually fund raise, they just pay the entry fee (plus an extra donation if they feel especially generous) which goes to the charity. Some folks are more active and actually solicit donations, but many - maybe most - charity runs only require an entry fee.
As a runner I sign up for these to give me some motivation to keep running and the entry fee/donation is just gravy.
People sponsor charity runs because there are people that enjoy running enough to pay to run a race. The sponsors then use that money for good purposes, hopefully. The people who enjoy running like the activity for various reasons: they like to stay in shape, they like to compete with themselves, they feel like they fit in with other runners and enjoy their company in an activity, some of them like to feel silly by running in ridiculous costumes… The list of reasons go on and on.
If one is inclined to nitpick various ways to raise money for charity, one can find something absurd in just about every scheme. For example, kids selling candy bars to raise money for school or whatever. Most of the time, the candy bars are shit. Nobody actually wants to go to a store to pay $4 for a chocolate bar that is chalky and worse than Hershey’s. But they will buy it for charity - what’s up with that?
If you don’t like running, hey, no problem. But is it really that difficult to understand that people like things that you don’t, like perhaps Hawaiian pizzas, golf, birdwatching, watching American Idol, or whatever?