Many small open source projects still operate this way and their community consists a small number of people who have fun hacking on a project together after work.
But as projects grow some of them acquiesce to end-user expectations and slowly turn into "organizations".
They have schedules and regular releases, commit to timely triaging of bug reports, provide forums for end-user support, publish status updates, respond to feature requests, write documentation, have a slick website with a nice logo, form committees and sub-committees to make decisions, adopt codes of conduct to try to deal with the jerks who invariably show up, file the paperwork necessary to deal with big donations, etc -- all the "necessary bureaucracy" that comes with being a large, reputable organization.
At that point you've basically added back all the unfun parts of working and turned it into a second job, so why not get paid?