I get your frustration. Why don't they care? Have you asked people directly what benefits they get from Facebook and why they won't move?

I expect you'll get the following, based on my experience:

1. Exposure - Most people can access Facebook regularly.

2. External Accessibility - Facebook can be accessed on multiple devices, by people who are not tech-conscious.

3. Reliability - Facebook outages are rare enough to be newsworthy, so no matter what conditions the school or the org is in, it will stay up and be exposed and accessible.

4. Internal Accessibility - Everyone in the org, even the people who aren't regularly involved in marketing or community comms, knows how to post to Facebook.

5. Cost - Facebook's non-monetary costs are subtle and mostly apply to private users. To any organization with a tight budget, Facebook and Twitter are godsends, because they don't need to have a P&L line that can be scrutinized and audited. I'm sure you understand as a volunteer how important it can be to dodge the accountants while getting messaging out.

Of course there's also experience, knowledge, and negative inertia built up over time. Until you can cover all 5 of those points at least, you don't have a viable option. Nextcloud is neat, but who will administer it without pay? Facebook runs the platform without being visibly paid by the school or the org. Facebook has widgets prebuilt to integrate with the website (that the org also outsources administration of). Nextcloud doesn't natively have that. Facebook is hosted on a massive network of data centers, Nextcloud would have to be run on one mistakenly undiscarded computer acting as a server in the basement. And it would have to be that way because the org doesn't have the budget approval for AWS or added hosting. And yes, everything will always come back to that cost issue. Until you can beat that, you have nothing.

Government employees will take it personal too. Like even just asking a park employee what herbicide they are spraying in a park on weeds they will get an attitude. Like right off the bat. They consider anyone asking, or challenging the methods they use as a crank, they commiserate about crazy assholes, think people are stupid, and all back each other up. All government employees back up each other and their decisions by default, whether it's supporting local cop abusing his authority or a school only using fb.

Any sort of effort to get them to change needs to be an organized attempt if it has a chance of success, as the psychology of these guys precludes individual efforts from helping much.