Scappy died at the end of Ransom and Scooby Chief
He was most likely murdered. I have a theory that he was in the witness protection program. This was his original life. It seems like he hung around people with mob connnections. He likely had to go into Witness protection where he went to live with all new people. By connecting with his old friends, he blew his cover, was found out, and killed. The rest of his appearances are his dying dream/ him entering afterlifeScooby and Scrappy Doo shorts all new scrappy doo and mysteries: Are all his dying dreams. Thirteen ghosts is his entering the afterlife. the direct to video movies are him in the afterlife with Dracula being God. Scrappy's eyesight is actually just as bad as Velma's without her glasses.
Scrappy's pretty young, so at least some degree of object permanence is in play, but it would still explain a lot about him, even if he is actually crazy strong. Even if he can see decently, at the very least, he doesn't rely on it much.