Because it's bad for society, and because there's a long since established moral precedent in favor of things that may make life slightly worse for any given individual but better for a large majority of everyone.
It's also worth keeping in mind that there's a massive difference between "rich" and "ultra-rich". "Rich" and "poor" are, if anything, closer to each other than either is to "ultra-rich", and spreading out the spending of that last category would dramatically benefit an overwhelming majority of people without hurting the schools of ultra-rich areas in a way that, frankly, anyone should actually care about.
That is a very big claim. It seems just as plausible to suggest that society will in fact be collectively better off if every parent gives their kid the best possible start in life, even if some kids will miss out due to circumstance.
I recall hearing from New Jersey and judges taking strict stances of equal outcome in education that found locality funding discriminatory, was that the poorer districts needed to be funded more per student to start to reach parity. Which makes some sense on some levels given knowledge and habits from parents transferring and would need more effort and funding to catch up because they are already providing more teaching just in a remotely middle-class concept.
That sort of equality is clear folly, like trying to force every kid into creches and separate them from parents. It might work with some hypothetical aliens but in practice for humans prepare for massive psychic trauma, inevitable abuse from caretakers, and frankly a well deserved outright insurrection and rebellion.
Don't get me wrong, working to raise the floor is a noble goal and makes sense just from rational selfishness alone. Throwing more money at the high end is 'dimminshed returns' but the money has already been allocated for better or worse. And well the obvious caveats of how the money is spent matters, there is no fixed true universal value per of any unit or commodity.
I struggle to see how this is wrong. It's not any different than rich people buying private education, though it makes it easier for non-rich folks to try and find the cheapest place in that district so they can benefit from the well-funded schools.
It seems popular to think that people should have equal opportunity, independent of any other factors. But from the perspective of a parent, why should I be disallowed from providing my own kids every possible advantage I can afford?