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.
My guess without looking anything up is, keeping in classic. The p90 is the older pickup and was used before they created the tuneomatic and they make them like that now for tradition and keeping that old feel for vintage vibe reasons. There is also going to be a particular voice to the not getting exactly perfect intonation that I'm sure some people want.
save