one doesn't have to go into a change room to quickly throw on a jacket for the few moments it takes to see if it's a good fit.

It's not like one has to undress. Jackets go on over clothes!!
It's not that diffiult to do a quick assessment. "OK...I'm wearing a light blouse and this is a little snug. Guess it won't fit over that sweater I was planning to wear with it."

It's common sense, really.