I hope I don't end up like one of those people. I've already expressed interest in a few roleplays and I'm excited! I'm certainly not going to back out.
I totally understand how it can happen. I do. I used to be fifteen, with a short attention span, with which ensued a spectrum of problems, from reading comprehension (e.g. getting bored and "skimming") to waiting "too long" for an RP's rising action after the boring shit like worldbuilding and character development. I would thus join RPs thinking I'd get something different from what I would actually get. And slipping away was easier than having to look people in the eye and tell them I was leaving their RP, because if I was leaving their RP, it was inherently implicit that I didn't like their RP, and I was also a pushover who didn't like hurting others. Uhh, you can see that has changed, lol.
So, yes, I used to do this. Then I grew up. I was always offended and insulted when people abandoned, ignored, or deceived me, certainly, but it took me a long time to realize that I was probably hurting other people by doing the same shit to them as others did to me: apparently just pretending to like their threads, and then deserting like it was all a great prank. Asking yourself "why" people leave always hurts more than any reason they can come up with, even the most bitter and vindictive. So really, when people do this they either don't realize at all that they're hurting others, especially such fragile and egoistic people as artists; or they just don't care.
TL;DR: Anyone who does this is either a child or a coward, or both.