Because being able to choose schools is an incredibly privileged position many well-meaning parents and their kids aren't in.
The better question is why the US education system has become a zero sum game. But don't blame affluent parents for following the incentives the system has forced on them.
Well, you're admitting there's no personal cost to your advocacy, which makes sense and I can't really argue you're not entitled to feel that way. Like in criminal justice reform, homelessness advocacy, etc. everyone is a liberal until the first time they get robbed, the first time someone smashes their car window, etc. Personal cost has a way to delivering reality to people who are disconnected from it.
> It's very much in my interest that education be distributed evenly
If there was no cost to the high performing students to keep everyone in public schools, I would actually 100% agree with you. But limiting the upper percentiles' outcomes to deliver almost negligible changes to the bottom percentile doesn't seem fair, either.
I'm not sure I agree with that assertion. But, if it is, my vote would be to fix that instead of throwing out the system. Public education works - it worked in the US for decades and it works in most of our peer nations.