There's a number of reasons. I would guess for most people here it's really about control of their data, which is a form of privacy. Making sure it stays on their network (ie: in their control) unless they approve it to go somewhere else.

There can be financial reasons (eg: backing up 10s or hundreds of terabytes to the cloud can get expensive), practical reasons (poor Internet access, especially internationally), latency/performance reasons (home automation). Sometimes you'll also get better interoperability with selfhosted stuff since exporting data is usually trivial and there's no walled garden lockin. And that's not everything, just a few reasons I can think of off the top of my head.

But you're right that some of these are often not the case. It can easily become more expensive (depending on how you account for things), it's definitely more work & it's never as easy as "just install and app and create an account".

Finally we can't forget that a not insignificant number of people here are aspiring (or actual) sys admins. This is a GREAT way to learn the trade if that's your thing.