Probably because it is quicker to align one wheel to the other, rather than align two wheels to a centred steering wheel.
I never really thought about it before, but I used to have a car where the steering wheel was about 10 degrees off after having the wheels aligned. I assume they used the nearside wheel as the datum and aligned the offside wheel. Unfortunately the nearside wheel was getting a beating when I was curbing it.
I heard this on the radio this morning.
Cracking track