This rule certainly isn't universal. In Canada (at least at my airline) there's no requirement for passengers to be seated at the window exit rows, unless we enter what we consider a "planned" emergency, some kind of issue requiring an emergency landing but we have the time to do extra preparation in the cabin. It's kinda like an extra in-depth safety demo. I've only had to do it once in 18 years of flying.

I've likely operated hundreds of flights (Dash 8 and CRJ family of aircraft), with window exit rows completely empty.