Same reason as everybody else: because everybody else is still there. I’m almost alone on my Mastodon, Bluesky, Threads, and Tumblr.
Almost no one has ethical ownership as a priority when picking a social media, the deciding factor is always who else is on there. Weirdos like you and me are way ahead of the adoption curve.
I believe that fediverse platforms are superior, they’re more structurally resilient and they will outlive Twitter and Reddit and all corporate social media; but that’s a bet on the long run.