There's a time limit for wishing people back, if I remember correctly. After a certain point they've been dead too long to return via the Dragonballs.
Yeah, but he means their spirits in Heaven/Hell, Hito. If I remember correctly, there were a good bunch of villains in Hell when Goku's fight with Buu was going on. Hmm...
Cell, Frieza, King Cold, Jeice, Burter, Recoome, Dr. Gero, Guldo... But,
Dodoria, Zarbon, Bardock, Android #19, Garlic Jr., Nappa, and many others, weren't seen there. It's possible they were around Hell, but Toriyama didn't want to include them in the story anymore... and left out their cameos or whatever.