Actually, up to the point that they switched to digital ink and paint (and I don't know what season that was), they WERE shot on film.
Thing is, they were not EDITED or mastered on film, but on SD video tape, so to remaster those shows for Blue-Ray they would have to dredge up the original film as delivered from Korea, scan it in at HD resolution, assemble it all over again, and re-do whatever post fixes that had been done to the episodes.
The only show that has bothered to do such a thing for home video remastering is Seinfeld.
Thing is, they were not EDITED or mastered on film, but on SD video tape, so to remaster those shows for Blue-Ray they would have to dredge up the original film as delivered from Korea, scan it in at HD resolution, assemble it all over again, and re-do whatever post fixes that had been done to the episodes.
The only show that has bothered to do such a thing for home video remastering is Seinfeld.