If you left all four tires in the same corner of your car for years and
years, some tires would wear out faster than others. Worse, some of the
treads in individual tires could wear unevenly, with the inside or
outside wearing out first. Uneven tread wear is unavoidable. The weight
of a car is not distributed evenly to all four corners. The engine, the
heaviest single thing in a car, is usually at the front, so the front
tires tend to carry more weight. The front tires also take care of all
the steering duties. Plus, when you apply the brakes, the weight of the
car shifts forward. Think about how often you brake while cornering, and
you'll begin to understand why front tires almost always wear out
faster than rear tires.
