To begin with, adding anything inside would mean increasing the cost making the car, reducing profit.
From an engineering standpoint, it's another failure point too, but blah blah blah, right?

Also, they aren't designed to have us poking our meathooks inside 'em, it just turns out that we do it!

It can always be added by us!

(I'm suprised at how much abuse my wires have taken WITHOUT breaking!)