A few reasons:
1: Never learned to do it.
Speaking from experience, my first RP was Stratos's own PSS. Thing is, that RP, as engaging as the world was, left little room for much interaction or independence in the beginning. So within my first RP, whilst I figured out a whole bunch of other stuff, independence was not one. Then the next notable RP I joined was Ymora, of which this was not the case. Independent action was encouraged, interaction between students. This, however, fell into two categories of "Go get your own pokemon" events or just plain PC interaction. Which at the time was easier to not instigate because, well, I didn't know how to do it lol. I like to think this issue was solved to some extent but still, erg.
So then you get people like me, not often taking the initiative whilst the players who would are the hosts or are otherwise engaged. So newer people don't learn much independence with their character either. Rinse and repeat until we have this situation.
2: Unknown boundaries.
Independent action is friggin hard when you don't know when you can or can't. Referring back to Ymora, half the reason I didn't drop Claude into more interactions is because it felt like it would break the other peoples convos if he just popped in and interrupted. It was hard to find that sweet spot of popping into a conversation smoothly because you don't have those cues that you would get in real life, explicit body language, no locking eyes and starting a battle.
Of course there are other boundaries. Namely where you can or can't go. Kuro did a good job of at least making it clear how you could explore in the Gaelech Basm exploration segment that was meant to happen in PMs. I at least knew where I could go. But it's easy to forget that there's a nearby forest for a player to waltz into at any time or that there's a certain character around. It's a case of leaving Chekovs tropes everywhere in the hopes a player notices and messes with it and broadcasting constant possibilities of what people can do.