IMO, it's not the number of running plays. It's plays being tied to formations and not including all possiblities. For example, all I-formation runs are not in all of the I-formation sets.
Teams get away with a few running plays because they run them from so many different looks. Madden's playbook format limits that.
I'm guessing this will come back when they're finally able to tier the defensive playbook. Last I heard, because the offensive assignments are tied to the defensive call, the pre-schemed defenses are necessary. Maybe it's the same for the offense... defensive assignments tied to the offensive play call.