Charter schools deliver results the same way that private schools deliver results: selection bias.

It's really easy to have good outcomes when you have the ability to curate your student population. And though charter schools are regulated to make it harder for them to curate their student population, the statistical evidence is pretty unequivocal: they serve different populations than public schools, and their "better outcomes" immediately vanish when you control for that.

So, what is the issue with redirecting funding from sucky* schools towards ones that deliver results**?

* Schools that teach the general population

** Schools that teach a subset of the general population that always does better