In a similar fashion, one could ask why doesn't anyone make a Blish delayed blowback device AR upper using monolithic aluminum uppers with MLok slots for accessories and steel reinforced inserts at the wear spots. With modern zero lube surface treatments, there should be no need for the lube pads that the original implementation of the Blish delayed blowback device (i.e., Thompson submachine guns) had.
And, of course, the answer is the same - the cost/benefit ratio is unfavorable. To wit, the British company, BSA, made a couple versions of 9mm (and other caliber) Thompsons after WWI, but only a few were ever sold and the manufacture of the firearms was discontinued.
MHO, YMMV, etc.