81k miles. Fourth set of rotors. What gives?

We will need more details before answering.

Disc brake rotors do not warp unless something causes it! ... Without personally inspecting the vehicle I can’t say what that cause is ... Improper wheel nut torque - calipers partially sticking and overheating rotors - inadequate rear brake bias - poor driving habits.... Recommend finding ‘experienced’ mechanic for *hands-on* evaluation - that hasn’t happened yet!