This is because shops usually hit that nut with the air gun and all the power it's got. They usually over torque them. Over time they seem to tighten even more. So unless you have a really good air gun with lots of CFM, the nut just won't come off.

I have a 3/4" drive power bar (30" long) with 32mm 6pt socket for such an ocassion. Put someone in the car with their foot hard on the brake. If it won't snap loose with the big bar, get out big-bertha hammer and hit the power bar HARD. This sometimes snaps it loose. If this doesn't work, get out Teddy-the-Torch and give it the heat. When you re-install, put on some "never seize" and torque it to the specified amount.