It's because, by & large, some people hate to think that some children are just inherently more intelligent than other children. The age at which they are tested and deemed 'more intelligent' is neither here nor there. They just are. yes, okay...you might get some late bloomers...but, thousands of children are perfectly capable of, and do, display sufficient levels of higher intelligence when they are 10/11.
And, as I have said a million times Tutors do not have magic wands. They need something to work with. We're in a GS area, and I know plenty of children who were tutored, who didn't pass the 11+.
And, yes...there are some children who somehow scraped through the 11+, and continue to barely scrape by at GS. But there's really not many of them at all. If there were then the GS wouldn't have such excellent exam results.
And, yes...there are some children who might really excel at, say, maths and science - but struggle with English. So, it's unfair that they don't get given a grammar style education. However, grammar schools are brim full of children who excel at everything (pretty much). Even if their 'weakest subject' they are still in the top 25%.
I don't believe that they are evil hot houses, either. The vast majority of the children cope absolutely fine academically, and are no more stressed than any other normal child at secondary school. They're certainly not all being tutored. There are upwards of 1200 girls at my DDs' grammar - and I can promise you, there aren't even a fraction enough tutors locally to service over a thousand girls 'supposedly' struggling academically.
They're doing fine. Thriving academically, on their own merit and ability.
My only argument against grammars is that they shouldn't cream off the top 25%. It should be more like the top 10%.