Why are jeans commonly used in weight lose adds?
Because skinny jeans (which is what I usually see on weight loss ads) are form-fitting and therefore it shows off how well they fit into them after losing all that weight.
they be look sexy
The jeans they use are usually skinny jeans and they're trying to make a point as to look at how what they can fit
Because shorts are too mainstream.
They are usually worn by many people.