Why aren't passengers trained and evaluated for operating emergency exits in commercial aircraft?
Former TWA here. 

Emergency exit seats used to only be given to passengers who are ready, willing and able to assist in case of an evacuation. They could not be assigned in advance, the passenger had to be asked and evaluated in person by the boarding agents, who would have been held responsible if a not-able passenger was to be seated in any of those rows/seats. This used to be the rule, it was deemed that safety was more important than selling seats. 

Maybe the rules have changed since?