They aren't farther out than the fronts really. Without spacers the front wheels were flush with the fender lip, while the rears were so far into the wheelwell that they would tuck with air ride. This didn't look good because it made the rear look lower than the front and it made the wheel gap look uneven. When the sun shone on the wheels, it looked weird because there was always a shadow over the rears. Besides, this setup is not unique to RWD cars. Early 2000's Mustangs have the same inset rears, while most new FWD cars have equal offsets with their fenders. Aparently auto manufactures push the rear wheels in to reduce understeer, which is most common on FWD cars.
mustang RWD: rear wheels farther in, very simular to Aleros
G6 FWD, wheels match offsets to fenders
Now I know what you are saying, you can see more of the tread on the rears, but this is because the front fenders are straight up and down, while the rear fenders curve in at the bottom.
here you can see that they are at different angles:
Its okay if you don't like it though. Oh p.s. The staggered look has nothing to do with how far the wheel sticks out, it is all about having a rear wheel that is wider than the front, usually with a huge lip.
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