Why? Because the low end, etc. has so many more customers that, so far with Moore's law, etc., the much greater investment in progress in the low end, broad market, common computer hardware wins against the much smaller investment in the high end.
Or, as was explained to me once, as maybe no more than a hypothetical example, invent a Lisp chip. Great. When running Lisp, beat the pants off all the general purpose processor chips. But just wait until the general purpose processor chips are a factor of 10 times faster and then lose out because the small volumes of the Lisp chip can't keep up with, say, Intel heading for 7 nm or some such.
Besides, for graphics, where get the big business volumes, for the high end engineering graphics workstations for high end engineering or for millions of bored boys not old enough to shave yet who just love playing video games?