We could afford it... but won't.
We'd rather have that money for things that will benefit the whole family a little more. Larger house, holidays etc.
For context, DH went to a private school all the way through. I just went to the local primary and the nearest state secondary. We're about equally intelligent I'd say. We had broadly similar GCSEs, I did better in A-Level, AND got a better degree grade. Granted, I probably worked harder, because that's just my nature. But still. His private education didn't do him any better than someone who did the state system throughout. I also have a better paid job now.
Our pre-school children both seem fairly bright and articulate, and there don't seem to be any learning needs. But if they appear, we'll support by using private tutors if necessary.
By going private... DH says you do buy yourself a bit of an entry to "a club", but membership of the "private school elite" only really pays off for a VERY small number of occupations. Mostly in finance (and then only in the city) or old-school corporations. For 95% of jobs, a private school education gets you no further, and can even stand against you in some settings, as they're seen as snobby, and like you got a free leg up.