I'm sure popular instances have setup Patreon accounts and the like for donations. Each instance is (essentially) being run by some person. It's just someone who decided it was worthwhile to set it up. They probably started with a small server in the cloud. At the start, it might have been free. But as more people join/interact, it starts to cost money. So the owner of an instance probably looks to the community to keep it running.
It's cheap to run, but it does still scale with the number of uses. So, it's very fair to have a place to ask for donations to keep an instance running without burdening the person who ownsit.