Probably a decent number of them like me where when I signed up there back then it was basically just lemmy.ml and lemmygrad.ml to pick from. Now that there are other options I created accounts on a couple other instances for redundancy (plus at the time lemmy.ml was buckling under the load) as well as having content and moderation rules I liked better. I’d bet a large number of those inactive accounts are actually active on lemmy, just on different instances.