Because transitioning is not humanly possible. That would be the worst thing to support ANYBODY. And, I wouldn't prevent any of my adult children from doing anything. But I'm DAMNED sure not going to allow a child who can't drive, smoke, drink or vote try to tell me some babble they heard on the internet was actually valid. This is why we HAVE parents and people to guide and teach us, so we learn in a safe environment without harming ourselves as children.
A child's mind is NOT mature enough to make ANY life-altering, permanent decisions. If you can't even get a TATTOO before age 18, you definitely can't chop off your penis and take hormones to fake being the gender you are not.