Sure, many people are fit enough to ride a bicycle for a bit on flat land when it is dry out. I'm not sure "most" people are, though, and I'm guessing this gets less the older folks get.
I do not live on flat land and I live in a place with winters. I, in fact, live in a mountainous area. Riding a bike is not an easy feat here - I've been testing this summer. Short slight inclines aren't too bad: The occasional steep hill isn't so bad. 15-30 minutes uphill, however, is too much for me. All uphill one way isn't easy. I'm not the only person that struggles with this: Biking forums are full of folks that struggle with uphill, and it takes time to get fit enough for this.
Electric bikes are the solution. I see more and more people commuting with electric bikes.
I've been commuting by bicycle for years, not for ecological reasons, but because it's convenient and healthy. That being said, I wouldn't do it if the roads weren't safe enough. To me, this is the main limitation. Bad weather and hills can be overcome, but I don't want to compromise with safety.
I do not live on flat land and I live in a place with winters. I, in fact, live in a mountainous area. Riding a bike is not an easy feat here - I've been testing this summer. Short slight inclines aren't too bad: The occasional steep hill isn't so bad. 15-30 minutes uphill, however, is too much for me. All uphill one way isn't easy. I'm not the only person that struggles with this: Biking forums are full of folks that struggle with uphill, and it takes time to get fit enough for this.