Why don't the P90 SGs have the tune-o-matic

I know the some do, but in general they don't. If you look here 5 of the 6 models have a compensated wrap-around bridge, and only 1 has the separate tune-o-matic bridge and stoptail. Then if you just look at all the SG listings the humbucker models pretty consistently have the two piece system.

But then when you look at P90 Les Pauls, it's much more mixed with both bridge styles.