Things need to be paid for but not everything needs to be paid for by everyone and most things are far cheaper than you'd expect. I played a MUD (and met my partner there!) for several years in college and afterwards. Initially I offered no financial support (since I was a starving college student) but when I had a job I sent in 10 bucks a month. That was a quarter of the cost to run a server, website and forum for about 120 people, we were generally overfunded but that was the cost of a wow subscription at the time and this was worth more to me.

Usually a few enthusiasts can just bear the lion's share of the cost to create the infrastructure for a community, excess can go to long term contingency funding and, in the unfortunate case that a community completely runs out of funds then it'll stop existing until people care enough to create a new one.

Video hosting and the like are dramatically more expensive but they can be reasonably subscription based (see Nebula and Dropout[2] neither of which have the VC backing to light piles of money on fire just to sustain a user base) but not everything needs such a high level of technology.

Heck, back in the day the majority of traffic that a website that was ad-driven needed to host was the ads - if you were half-decent at writing asset caching rules images became a non-issue that were usually handled by proxies/other intermediaries.

Everything costs money - but it's important to remember that a lot of services charge a lot more money than they cost to run and that ad money is a lot less money than most people realize[1].

1. A big exception to this being things like newspapers which really are in a hard place. Their expense isn't in hosting or other technical doodads (e.g. the NYT Crossword puzzle) - the subscription you're paying is to afford the huge team of reporters and editors that are needed to produce the information gathering and presentation.

2. Edited to add - Dropout is probably a terrible example here since it's a lot more like a newspaper, only a sliver of the cost is technical, most of it goes to the production team and talent they're retaining. But I'll leave it in there unedited.