The difference between self-hosting most things, and Twitter and Reddit (and Facebook and Slack and Discord) is the network effect. If I wanted to self-host my pictures that I share with friends, I can still just send them the URL. They might be annoyed that they're not on Instagram and have to use a web browser instead, but the people that want to see how my long weekend went will go see the pictures. To self-host something like Reddit, I need to convince other people to change their habits and their choice of platform. As not-a-million-dollar-corporation, my ability to have a polished UX is rather more limited, so I can see why someone would go back Twitter and Reddit.