Because they wear those jeans for every day life and also ride a bike. Instead of having extra kit to change into every time they get on a bike. They just get on the bike and ride.

I wear "skinny" jeans because I am a skinny person and I feel they fit/look the best. I've also invested in nice pairs of jeans designed for cycling. Couple percent of spandex or similar material in the fabric. Allows the jeans to have a lot more stretch. They also have a higher back to avoid the dreaded crack! Reflective strip inside the pant leg so you can roll it up and be more reflective. Double gussetted crotch to keep them from bunching while on the seat etc... This way I can hope on a bike at anytime and be comfortable. Want to get dinner with friends 5miles away? Not a problem, no need to change into kit.