Because it's unnatural and contributes nothing to the continuation of the human race.
What happens to "pedophiles" in the wild?
>Attracted to children
>Have sex with child
>Unable to procreate
>Genetic dead end
If the person/animal continues to have sex with prepubescent children then literally nothing happens. Their bloodlines ends with them. Problem solved.
Adding stuff like
>b-but the child's well being
>b-but it makes them happy
are moral arguments that only apply in social constructs that can shapeshift depending on the time and place. But at the end of the day having sex with a child that is unable to bear children contributes nothing.
You can be a moral relativist all you want but at the end of the day if you aren't having children then you are irrelevant in the grand scheme of things. That is literally your only job as a human and what drives us as a species. Sex is the road to reproduction. Reproduction is how we continue our species.
We can all act like things like homosexuality, pedos, and other sexual activities other than hetero-normative ones are a-ok in a modern moralist sense but if it came down to the nitty gritty of keeping our species alive (let's say there were only 50 people left on the planet) these activities and attractions would do more harm than good.
Thankfully we have billions on this planet so a few percentage of them not breeding can be excused, tolerated and even accepted socially.