Because we need a system that works for all! Or at least MOST children. I wouldn't agree with a system that just benefited lower attaining children either - we've done grammars before, they didn't work then and didn't work then, the ones we have left don't work, why are we flogging this dead horse? Let's try something else.
I think what we need is more vocational options for children to choose, (key word choice, not forced upon them if they fail a test at 11), vocational things that can be done alongside a smaller number of GCSEs, much greater involvement between universities and secondaries. I agree we need to continue to boost those working at a higher level, but feel this does not need to be done by sending them to a seperate school. What exactly is it about the grammars that you feel benefits highly attaining children? Let's implement those things into a comp and make it accessible for all.
But these options should be available to all, and not limited based on the results of a test at 11.