I ride because I live 800 miles from the nearest mountains. I'm actually a hiker and a skier, but one or two trips a year doesn't cut it, so I got into biking as an alternative. It gets me outside, and we have a nice wooded bike path about 30 miles long, where you don't have to watch for cars.
Plus it's good exercise. I don't like being sedentary, and like being fit and having the added benefit of the endorphine / higher oxygen content highs that come with exercise.
If I lived in Colorado, I wouldn't bike very often. I'd be in them thar mountains hiking and skiing all year round. I like dirt and snow better than pavement.