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.
