I think it's that you get polarised views on Mumsnet in terms of the people who choose to post. The vast majority of people in the country aren't agonising about school choice, either because there's no point as there isn't a choice, or (far fewer here) because they don't care, or because (in most cases) they're perfectly happy with their school choice.
The most vocal posters will be those with a particular axe to grind, a particularly strong social or political conviction about education/grammar schools/private schools, or a particular choice to make.
That's why there are a disproportionately large number of threads on here about Eton or Winchester - not because most posters here have ever considered those schools, but because for the tiny minority thinking about it, the choice seems very high stakes.
You also get anomalous posters on here like me - I'm strongly opposed to grammar and private schools and any kind of religious or academic selection on principle, but am not very principled in practice, given my choice of schools, and so have sent my children to a selective private school.
People also post blanket statements like "Schools in London are so much better than the rest of the country and get much more money" which can irritate and inflame. Not all schools in London are good or outstanding or are particularly well funded - it varies by local authority.