Most Helpful Guy
it's a scam by pants companies to get you to unintentionally show off your goods0
Most Helpful Girl
It's true - it's about the stress on the seam getting tugged in opposite directions with each step you take. You can find pants with a gusset at the bottom of the crotch seam (a diamond shaped piece where otherwise the four pieces of the pants would come together under your seat) that eliminates the problem - typically in cargo pants, military surplus, or good hiking gear, but they're mostly made in men's sizing, so unless your waist and hips are very close in size, you'll have to hunt for women's. It's also the case that plain weaves (threads alternate one over one under) are stronger against that kind of tearing, so maybe you want to buy pants that aren't denim. The twill weave (that gives denim its diagonal stripe appearance) makes a fabric that is stronger against abrasion so it lasts much longer if you work in your pants instead of just walking in them.
There you go - way more than you actually wanted to know. (Guess what I studied in grad school!)1