Same reason people buy land and build a house on it: It is in their control. If I host my content on some plattform, I have to bow to their TOS/Tracking/Changes and am more like a tolerated tennant. But they are the HOA that can tell me what they want and do not want regarding content, frontend and backend. I also most possible have to allow them to use my work for their own benefit. If it is free, maybe they profit from you.

People do not get that having your own URL, your own Email and your own Websites, was the most common thing in the past. I staked my claim like the settlers going west. I own my acre of digital Land. I am free to do what ever I want on my property and nobody can take it away as long as I pay 5$ a year for the domain. That is 50$ in 10 years. That is 400$ for my whole life. For a place that I call mine. My digital Landscape. This is my space. There are many spaces out there, but this is the acre I am gonna develop.

Of course I could rent a room in the city by giving away my privacy or the control. And there are many good reasons to do this. But having your own piece of the internet is what makes me feeling more like a part of that net and not just be a cow in someone else shed.