Because when it comes to state schools, there will be children there from all faiths and none. To ensure equal treatment, no one belief should be promoted as The Truth. If you're going to have religious schools which teach otherwise, they should be private schools.
How do you feel about state schools with multiple chaplains from different faiths? What if every family there is from a particular religious community - would you be ok with it just being one chaplain then? Or do you have a fundamental issue with state schools taking or presenting any particular religious belief beyond a sort of vague secularism? You said upthread I think that your children had gone to a CofE school - so you're ok with religious institutions being involved in and funding schools?
My own kids go to a 'non-denominational' school which, being in Scotland in the area where I live, is by default a Protestant Christian school. I don't particularly like that approach (at least be honest!) and I'm not Protestant myself, but eh, I chose to send them there.
I think there's a fundamental divide here that we're not going to get past in how religious people see religion. If religion is important to you, it's not important in a way that you can just do it privately in your own home and parcel it up into a few hours on a Sunday (or a Friday or a Saturday or whatever) and not really talk about it or do anything noticeable with it in public. It matters for your whole life. It's not a case of "ah well, if my religion has to share space with others then we need to discuss them all only in a very secular distanced way" - there are religious people who would choose to send their kids to the school of a different religion rather than one that took a secular approach.
If your approach is that it's simultaneously sort of harmless and irrelevant to daily life, to the point where it can be tidied away into a few private hours, and also so dangerous that children shouldn't be directly exposed to it, then you're coming from a perspective that not everybody shares. And you have every right to that perspective, but again, people who don't share it aren't going to necessarily agree with you that the entire country should be run that way.