Apologies if I am repeating what others have said upthread...
But if you reward, incentivise and severely punish schools on the basis of A to C grades, consistently, over years, then schools - however they feel, morally, about it - will work to maximise A to C grades. Disproportionately, the schools that struggle most to make that benchmark will focus most on it.
Schools far, far above the benchmark A* to C grades have the luxury - and in these days where failure to meet flor targets means sacked headteachers and forced academization, it is a luxury - of devoting some of each day's 24 hours per teacher to the stretching of the most able. Most MN-popular schools will have that luxury, many other schools do not.
You cannot simultaneously put extraordinary pressure on schools and teachers to do Task A (get a certain proportion of children to jump through Hoop 1) and expect them, by the way and unrewarded / unrecognised, to also do task B (ensure a certain proportion fo children jump through a different hoop, Hoop B). When the jobs and livelihoods, mental and physical health of teachers and heads depends on the A to C percentage (and the Ofsted grades that are ever more tightly linked to these), then funnily enough, that's what they concentrate on. In some schools, where due to the luck of the cohort that pressure is felt less keenly, then there is sufficient time, enough effort, and enough resources to also ensure that the most able students get As.
People, in all walks of life, do their best to do what they are asked to do, what they are rewarded / punished for and what is valued in the environment they are in. No-one should be surprised that in a punitive environment where A to C percentages are the be-all and end-all, the A/ A percentage is not ocused on to the same degree. If it becomes something that is highly valued, then in those schools with resources available, it will be focused on. In those schools very near the floor level for A* to C, where there are quite simply no more moments to be wrung out of every day or hours out of every teacher, it may be focused on less - not due to the will of the teachers, just due to the physically and mentally impossible demands bein placed upon them.
On a personal note, DS's (very MN) non-selective is supurb at stretching pupils. But it has the luxury of a very able cohort.