My Crotch

In the past 3 weeks I have ruined two pairs of khakis and a pair of bike shorts in the crotch. I have been wondering why the crotch of my pants is always the first thing to give. Is it the friction or is it under too much pressure? And if this is such a recurring problem then why aren't pant-makers doing something about it? I would not be that hard to reinforce the inseam of pants ... or am I overlooking some complication. It seems to me that science should be focusing more on this question.