The lack of knowledge, which others here already pointed out, is only one part of it. I have a knowledge due to my education and work, but I really don't want to. I selfhost the stuff, that I use personally and locally and access it through a VPN. But a ActivityPup Server, that would need to be reachable from outside, I really dont want to bother with.
Just doing a port-forwarding, like you suggested, is not a great idea security-wise, if you dont take additional precautions. You'd probably want the publicly hosted stuff in a completely different subnet from your LAN, that all your other devices use. Or at least have VLANs setup. All of which is not easily possible, or even impossible, with the standard consumer networking hardware most have at home.
Additionally, you'd need to take care of system and application updates, backups and so on, which takes a significant amount of time.
All in all I really don't care enough about social media in general to justify the hassle.