I think the decision to limit sliders so severely is a tacit admission that they don't know well enough what does what within the game.
It's clear that certain sliders for some time have had lots of unintended secondary effects, likely stemming from the fact that the game hasn't received a totally clean overhaul in a really long time.
If it's obvious to us that several of the sliders we have effect too many different things, then maybe they're not capable of creating sliders that only have one, specific, precise effect at this point. Like an interception slider that ONLY modifies the attempts at picks and their success rate. Or a tackle slider that ONLY modifies the chance of making or breaking a tackle.
To get more sliders and more precise delineations of the effects of those sliders, they'd likely have to start fresh code-wise, and that's not happening any time soon.