The one time I ghosted someone was when I was (well, still am) mentally deranged and I smashed my phone one day - thus severing all connection with the person who I wasn't really into in the first place but was just going with the flow (met her on ok cupid). Other than that, a few people have done this to me, I assume because they 1) can, 2) don't care enough, 3) are afraid of hurting feelings, or 4) something else. If you have some a fair amount of interaction though, you really ought to let the person know. It's called ghosting for the reason that it's literally like them becoming a ghost, and I assume most people aren't interested in having ghosts come into their lives.