Personally, it's because I want to document all (like-ly) personal things, and they need to be easily accessible. Sure, not everything (especially the very private stuff) can be posted, but I still want a space to speak (write).
I also enjoy hoarding things I like that I come across on the internet or from other sources (besides the internet).
And I'm pretty lazy when it comes to social media timelines these days — they're full of algorithms, like everyone's competing to do this and that. Of course, there's always a slight urge to join in, and I do follow along sometimes, but then after a while I get burned out because of one thing or another. Tired.
Finally (though I haven't experienced it yet), it's about freedom. We know that social media has owners, and all the content you and I post isn't ours — it's theirs. And as owners, they can delete or suspend your account or posts however they like, without notice or any clear reason.
So that's why I want to have a blog.