Manufacturers do sipe their tires. The Pro-Comp A/T and BTG A/T KOs (I have the BFGs) both come with siping.

BUT, the siping you get from a tire installer is more effective, because of the nature of the cuts they make. Manufacturers have to make the sipes in the tire mold, so they cannot make as many and they have to be wider, so it comes out as a gap. The siping done by tire installers creates no gaps in the tread, they just cut the rubber. Much more effective on ice, like thousands of little teeth.

That said, when I got my BFGs I did not have them siped because I lived in Cali at the time. I highly doubt that siping extends the tread life... I would guess (not a very educated guess) siping creates more friction and heat on dry surfaces, reducing treadlife. Just a guess.

-Daver