If your streets are like the ones I often see, it is no wonder. Shock loads on the ball joints from uneven pavement are rougher (because of speed) than off-road trail riding.
I work on a seminary campus and part of my supervision duties are the fleet for the campus. We regularly replace ball joints, tires, brakes, and even axles and gears on vehicles (mostly Superdutys) that have less than 20K miles. The campus is full of speed bumps and it just wears out parts at about 5 times the rate of highway use.
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