Huh? Why don't they exist in ROTS? Because they weren't mentioned? That's a really literal way to look at fiction.
1) Dooku has no apprentice in ROTS - not Asajj, not Savage Oppress, etc. You don't just quietly retire from the Sith. She is absolutely dead IMO, because Palpatine would never be stupid enough to have another Sith adept wandering around out there plotting her own agenda. (The whole running away and hiding thing is weak IMO because Palps would still use his Sithy powers to hunt her down and kill her)
2) Cad Bane is seemingly involved in every Separatist plot and personally works for Dooku/Palps on a number of occasions. It's not like they ran out of money or anything, he's a major player and his complete absence by ROTS indicates to me that he is no longer a factor of any kind at all. Again, not the type to just walk away.
If you saw the first half of TPM and then went straight to AOTC, you'd be wondering what happened to Darth Maul too. You'd have to assume he was dead, IMHO. Otherwise he'd be on screen. Same deal.
Just a constructionist viewpoint...a logical short-cut, as it were. Obviously the real answer is that none of these characters had even been dreamed up yet when the PT was filmed...