Every time you ADD a new potential part, you have to go through validation/certification...and possibly even crash testing, depending on the parts/system affected. Which means time and expen$e...
"Why GM"? Warranty...and the wish for most NOT to "mess" with their new car... Heavy duty mechanical unfits on Brand New cars is clearly NOT the majority's wish.
The Brembos are already "validated/certified". Bars and bushings are do-able, quickly. Perhaps springs, too, although a "drop" should NOT be expected (crash testing of new bumper heights!). The aftermarket already has THIS item...in spades.
Combined with the RS 20s on an LS/LT with it's lower curb weight, it's "1967" all over again (the original Z28, NOT the quickest/fastest, but certainly THE most "drivable"!)!