London is very competitive.
There are lots of alternative options obviously, but people also want their children to go to the best school for whatever the next steps are.
London also has a high proportion of children in private education.
When you have state 6th forms asking for those kind of entry grades, they are likely to have sizeable cohorts chasing offers from very competitive university courses.
I have a child in state in Y11 and am talking to lots of other parents in the same cohort who are applying to the schools like King's Maths School, BRIT (up to 2,000 applying for 50 places on some of their courses) and the most selective 6th forms. These parents are state and private - and people are very aware that competition is going to be more intense this year than last.
It's not a case of thinking the private students are cleverer or going to have higher grades. Any student coming out with the scores needed for these schools is clever. It's about the increased numbers fighting for the same spot.
Bit like super-selective 11 plus where it's not passing the exam that counts, it's passing with a score high enough to be in the top 120 students who took the exam.