Yeah, it's not like "write with everyone!" is a magic bullet solution that'll usher in an era of untold prosperity. But there are reasons people flake/ghost, and it's not always because they're wee cunts.
I have been on the side of "ghosting" a site. I'll join a site, write a character, invite people to plot with me, get three or four people to write with, and at some point coincidentally all of them get busy/in a rut and cannot reliably post anymore.
I don't want to take on more threads and wind up in a situation where I'm overwhelmed, because those threads haven't "ended". I also don't really feel like waiting two, three months for my writing partners to return. In those cases, I just quietly dip because I don't want to cause unnecessary drama either by calling them out for posting elsewhere, or by moving on with other threads and getting all those nasty "Traveller isn't interested in our thread anymore!" accusations behind closed doors.