Because mental health requires certain conditions, and one of those conditions is each phase of life playing in the correct order.
Sexual interaction with a child screws up their development, because they try to absorb the state of mind of the adult but they can’t because there’s nowhere to put it. So they pantomime it, and it stops them from interacting with the things appropriate to their age. They stop developing, because their energy goes into maintaining this mask.
The things they don’t develop include boundaries, a sense of self, the ability to trust other people, and any opportunity at developing a healthy, normal relationship with sex when they grow up.
It’s somewhat similar to what happens to a child when they are burdened with emotionally caring for their parents. Except it’s more intense, with older parts of the nervous system getting mis-developed.
Think like a baby in a small metal box. As the baby grows, if the box stays there, the growth of his body will be stunted and fucked up. And it’s not like removing the box later allows him to pop into a normal shape. His physical development is ruined. It won’t get back on track.
Sexual interaction with a child is like this metal box, except it’s a behavioral box. It’s a mask. The kid is participating in something he or she doesn’t have the mental configuration to engage with properly. But children absorb patterns from adults around them, and so they take on this pattern, and it’s the wrong pattern for that time in their life, and like the metal box around the baby, it fucks up their mental and emotional development.