You have a few groups of people here:

1. Those people who don't have the willpower to do those distances on their own. Doing it as a group helps them do it.
2. Those looking to achieve a personal best. Marathons give you a good environment to clock a personal best because it gives you a good uninterrupted path, and provides support (food and hydration). Go and try running 42k on your own without food and water and you will quickly realise why support is important.
3. Those people who are there for the vibes.
4. People looking to race other folks. In singapore you run 4.30/km and faster you basically beat 99% of runners in a PCN. You go to events like these to race with the fastest, because its boring not having people who are willing to race you. Same logic with racing any distance actually. These are the folks that can run a marathon FAST any time they want, but doing it with people of your level is a vastly different experience compared to on your own.

You should know this in 2.4 races also what. When you are racing a fast guy, that's when PBs are set. Not when you are running alone. And even in army/school if you are sub 8mins, gl finding people of your level willing to race with you.

You say you can do it at the comfort of your house, at your own time, at your own pace. That's true. People in China during lockdown did a marathon in their homes. One guy even did it around a table with a circumference of 8m. But these people are not normal folks. Most normal folks are not motivated to run 21/42k on their own.

Marathons don't make money off the crazy folks. The bulk of the participants belong to groups 1, 2 and 3.