Count Gensokyo
Super Smash Bros. Melee is a Fair & Balanced Game
I think the main reason can be boiled down simply to the fact that it's difficult to reject people. Whenever you lose interest in an RP, sometimes it really is just you, and sometimes it really is just the other person (or people). If it's the latter case, to actually tell them you're done with the RP because it isn't interesting to you is hard, because in a way, you're telling them that they just weren't good enough for you. Maybe that isn't always the case, and there are certainly times where you've just lost interest in the subject matter through no fault of the other. It's often easier to just ghost someone rather than telling them you don't want to RP with them anymore.
I'm of course guilty of having done this in the past, but now I think I do a fairly good job of avoiding it. The main reason being that you will rarely, ever see me take part in an RP that doesn't have one person acting as a dungeon master. The fact that some of the people reading this post are looking up "what is a dungeon master" is testament to the difficulty of actually finding someone willing to play that role. Because of that, I'm motivated to lock down whoever is willing to play that role for an RP. I'm currently in one RP where my replies slowed way down because I just wasn't too interested in how the story was going. Instead of just ghosting it, I took the time to talk with the DM so we could work out ideas on how we could make the RP better for the both of us. I think for most people who do the more common type of RPs, it's easier just to find a new RP instead of trying to work things out, but for me, finding a new RP can take weeks or months. In fact, the very first search thread I made on this account was only turned into an RP years later (thanks Count Gensokyo , love youuu). So, I'm always leaning towards the latter option.
At the risk of sounding pretentious and waxing philosophical while not knowing shit about anything, it's human nature to reject others. It's just that a lot of us don't like confrontation. But for me, I don't care too much about confronting people or telling them what I think needs to change. Probably because I'm an asshole. But, still, I think that's better than disappearing without a word. If you can't resolve things and end up dissolving the RP, that's okay, at least you didn't leave someone hanging.
Dangit man, you made my day, lol.