In my expierence most teams don't have great passing and offensive flow so their team doesn't get many assists.
A few more reasons
-really good teams have high passing on their SG, SF, etc. These players will in essence "steal" assists from the PG.
-A PG with shooting equal to passing will probably shoot as often as he passes, taking away some assists. And if the players around him are really bad, he could even take the lion's share of the shots.
-People have over-developed outside d and under-developed passing. The general BB philosophy (as I have heard it) is to train outside d first, everything else later. Since outside d is what stops assists, is it any wonder that they are (a bit) lower than expected?
By the way, things used to be worse than this. I really like the way the engine works now in terms of passing, I do not think we need an assists tweak again.
Run of the Mill Canadian Manager