Same as other folks have already mentioned.
I participate in many small to medium size group rides, but these are all no-cost and mostly informally organized. Even our local version of the monthly last-Friday Critical Mass is more of a casual social group ride. We seldom draw more than 50 riders in the best weather, and some of our wet winter rides have had as few as 3-5 of us.
And there are several fast club rides a week, from road to gravel. Groups are usually manageable, from as few as three of us up to a couple of dozen. No cost, nothing official, just a regular show up and ride thing.
Cost. I can't justify spending money for most one-day event rides. The last such ride I did was probably the old Tijuana-Ensenada (before it was the Rosarito-Ensenada). Fun, huge crowd. I'd do that again.
One of my pet peeves is the so-called "charity" or fundraiser ride/run/whatever. It's vanity. Especially the illness-specific events that are promoted way out of proportion to the mortality rate and incidence of the disease. It's like the old Jerry Lewis telethon for muscular dystrophy. That's one of the rarest of diseases. The telethon mostly served as an ego vehicle for an aging former star who became insufferably maudlin. Most such charity/medical issue sports events profit the organizers. Self-congratulating participants get t-shirts. The supposed victims get a pat on the head.
Wanna really help your favorite charity? Give them the money directly. Or ask what material donations they actually need today. Food drives are often well meaning but misguided. An efficient charity can make better use of cash, going through purchasing agents, getting discounts for bulk purchases, etc. I give food directly to people whom I know can use it -- neighbors, or homeless folks I meet along bike rides. I'll buy them something to eat, maybe offer a little cash. But charities, homeless shelters, etc., often need money more than physical goods.
On the other hand, some children's hospitals readily welcome new clothes, toys, games, even stuff for moms and dads who may be going broke from hotel stays in areas where there's no free or affordable housing while their child is in hospital.
Wannabe racers messing up a charity ride for other folks. That's my current peeve. If there's no sanctioned race, no primes or prizes, no emergency medical crew intended to handle race-type crashes, it's damned inconsiderate. There are plenty of venues for racing or just fast club rides. A charity/event ride is not the place. I know of a couple of local casual cyclists who decided not to participate in a popular upcoming event ride because of racers. Can't say I blame them. If they want to start early to beat the heat, they risk being overrun by wannabes blasting by in a 30 mph paceline. If they start at the back of the pack, they risk missing the cutoff time. Heck, I like riding fast, but charity/event rides ain't the place for it. Unless the ride organizers specifically permit racers and organize the route to protect others, and have appropriate first aid available, it should be a no-no.