I think fear is a huge driving factor in the Grammar system and the way it has become so tutor led. People fear comprehensives and they fear their children being in classes with disruptive children, because much of the teaching is by mixed ability.
It is true that a top set in a Comp can be as good and as high achieving as a Grammar,mbut there is fear that in a large Comp, your child might not make it into the top set and be left lower down with the disruptive element. And there is fear that there isn't much setting going on anyway - it is increasingly out of fashion and even if schools set for maths, many don't set for much else if anything, so this fear of a disrupted learning experience, or slowed pace is very real for lots of parents, even in good Comp areas.
There's also fear about things like a lack of triple science for all or limited languages, or music provision - things the middle classes love and would hate to think some children are getting elsewhere, but their children aren't getting.
And there's fear that the local Comp isn't even one of the very good ones that people always mention, but just rather mediocre.
On balance, I believe that the Grammar system is damaging. It is divisive, it creates stores for young children and their families and the testing methods are not foolproof. Most importantly it fails well over half of children and assigns them to schools without the respect of many and without the full ability range. This is all true. However, when Comps are so variable and even the good ones don't seem to separate the able or well behaved from others, many parents are never going to be happy with a Comp and will seek a Grammar - this is the reality. People have a fear about Comps, not just in terms of end results, but in terms of the learning experience too. It may well be that being educated amongst a broad range of ability and affluence can lead to brilliant results, but I think the key thing for many people is that they worry about their children's education being negatively affected by poor behaviour....and yes, of course poor behaviour isn't confined to lower ability children or those from deprived areas, but in many parents minds, there is a strong correlation. In the end, people choose not what is best for society overall, but what they think will be best for their individual child, and most of the middle class would in all honesty prefer their children to be educated with well behaved and clever children, if at all possible. That others may lose out from their choice, is simply not their key concern. Comps just seem a bit......risky for many people and where people do have choices (and money and sharp elbows can get you some choices about where to live and which schools are available to you) then they will.
If Grammars didn't exist, as they don't in many areas, people would go to Comps. Here we often hear from parents whose children did well at the leafy Comps in the top sets. I would like to hear from parents whose children went to Comps and were in the lower sets, or who went to the less successful Comps and hear about their experiences and if they were pleased. It would be nice to think that there are parents in all schools who are pleased..but we never seem to hear from those if they are out there, and this further fuels the urgency some parents feel to seek selective education.