Because mobile notifications require integration with operators and cost money.

This is not about the ones that are pushed over IP, this is about mobile push.

It doesn't require operator involvement, you can just integrate with FCM and APN?

FCM and APN are the operators

Mobile notifications do not require outside operators. UnifiedPush / ntfy is FLOSS, and allows for a single background connection for multiple apps / notification channels. It can also be self-hosted (and I do!)

This puts the operational costs (number of devices and notifications) on whomever is running the server - and because of how valuable metadata is, I expect them to be run in-house by governments

Well I guess you can do the same with zulip, redirect notifications somewhere else that is. Never hosted zulip, but it should be more or less staightforward when hosting it yourself, the 10 free looks like a limit when using their endpoint, not universal.