In my experience this means they're fitting wrong and there's undue stress in the place that they're failing. For crotch area specifically, look into Duluth Trading's "Ballroom" pants.
Alternately, the moment you buy new pants, add some iron-on patch reinforcement across the area. A two-dollar giant patch can spread the force out quite a bit and probably double the lifetime of the unreinforced fabric.
But really, a tailor should be able to tell you what's wrong with the fit and how to fix it.
As someone who has also had these issues with the crotch of pants wearing out, I would also suggest trying these options.
It's a combination of stretching and friction. Jeans have it especially bad because they have prominent seams right where the thighs touch, causing an extra large amount of friction. A gusseted crotch (which I believe the Ballroom jeans have) both helps with the stretching and moves the seams around.
Alternately, the moment you buy new pants, add some iron-on patch reinforcement across the area. A two-dollar giant patch can spread the force out quite a bit and probably double the lifetime of the unreinforced fabric.
But really, a tailor should be able to tell you what's wrong with the fit and how to fix it.