Probably in part because controllers still have to keep the aircraft separated before the fix, and letting them achieve the crossing time however they want will result in conflicts before hand.
Second, from years of difficult experience, if there's only 40 miles to work with, and a line of aircraft to sequence, speed alone won't do it, ever. The situation calls for vectors, too.
That type of solution is perfectly workable in a more strategic setting, but for down-and-dirty tactical sequencing, it's just not going to work.
Since you directed this at US controllers, is there somewhere in the world that does do this, successfully and on a routine basis?