I have done the whole ignoring thing various times. There isn't really a universal reason to it, it's mostly a case to case basis for me.
Sometimes I message somebody for an idea and during the plotting phase I might feel we are looking for different things, or aren't on the same wavelength. From there I have two choices then.
Either openly refuse, which makes me feel bad since I don't want to hurt any feelings, and also frequently results in my partner growing fed up or lashing out or getting stalky; or ignore and hope they simply forget and move on before we get too invested.
I have also left rps right after they began. Mostly if I, again, feel that the writing style isn't really mashing well. Because ultimately, when writing an erotic story, I want it to be a partner whose persona or writing I find attractive, inspiring and am willing to invest into. So if I feel we aren't compatible after it started, I rather slip out before we get too invested in it, because I don't really want to go ahead and be like "Hey sorry but your writing doesn't really appeal to me", which I find even more rude and arrogant.
Then there also are situations where I put up my own search thread but don't respond to some of the pms I get. Either because I saw their own search thread previously and already know we are looking for different things, or because I get an interest pm the complete opposite of what I'm looking for:
Me: "Hey folks, hoping for some emotionally complex non-con story rooted in the real world and more of the short- to mid-term length."
Potential Partner: "Hey, I saw your thread and have this plot of mine which is like a super awesome consensual epic fantasy long-term story you'd totally dig."
That's kind of missing the point of why I put up my own thread with the ideas I'm interested, in the first place.
The most selfish reason for when I ghost is when I feel really shitty irl and never actually have the energy for a rp to start with and just want attention and positive feedback for my ideas and opinions during the plotting stage.
Then there are also instances where I really adore a story and have a post perfectly mapped out in my head, but at the same time feel somewhat overwhelmed by everything I plan to touch on that I keep pushing the post ahead of myself with new excuses but never actually get to writing it. Lost much too many good partners that way.
So yes, I'm probably one of those ghosters people complain about haha. I'll say in my defense that, while I'm very cut-throat and selective about who I want to rp with, if I do get invested with a partner on the same wavelength as me, those rps tend to be fantastic with a possible long-term rp partnership forming.
Funnily enough, when I'm getting ghosted I get very irritated and start bugging my partners too. Hmm, I'm weird.