Saying it as the person in charge of developing Mastodon, probably the most likely reason more servers don't charge for access is that it's not built into the software, so it would require either customizing the code or manually sending out invites based on external payments (e.g. Patreon, some servers actually do that). I would imagine the other likely reason is that complexity of signing up as a business with your local tax office, charging the right taxes based on where the customers are located, and accounting. I do absolutely agree that paying for your account would make running the servers so much more sustainable.