It's because people always want to lower CPU QB Accuracy right out of the gate to get more missed throws. QBA is not just accuracy, but also impacts the kinds of throws they attempt, their pocket awareness, WRs willingness to go up in the air for passes, and even playcalling to a degree). We have seen this with other EA games too; this one isn't much different.
I am playing with CPU QBA at 39 in my slider set (normal speed) and I see better pocket awareness, willingness to go deep, and more varied play calling. With the right pass coverage settings, Completion percentages will stay in-check unless your defense is out-manned. Most slider makers want to automatically drop this to 6 or 8 right away and then wonder why the AI was so crappy....I used to do the same thing.