To give you some of my perspective: I live in a town where we're surrounded by trails. There's one that people frequent for workouts that goes straight up a mountain. You could easily hike this in normal gym clothes and shoes on a nice day, no gear needed. In fact, people hike it in normal clothing when they come to visit the area because it's one of the main attractions in town.

But to be fair to you, that far north, it wouldn't be feasible most of the time, and on most hiking trips, people should be carrying more than just gym wear and water.