Think of the cost of building all those mockups for the gate areas, and the pre-boarding tension in the gate area that would come from people practicing, noisily and visibly, for a dire inflight emergency.

It's far cheaper, and probably more effective, for engaged FAs to size up the exit row pax before pushback and judge that they're capable. Airlines -- and FAs too -- are all over the lot on this point. Sometimes a really conscientious FA will come back and interview all exit row occupants at some length, which surprises and pleases me. But at least half the time FAs do nothing noticeable.