The first rule of medicine is to do no harm. It's an ethically grey area to intervene with something the body is doing naturally that isn't putting the person's life at risk. The technology available today for gender transition is crude compared to what will be available in the future. I know intersex people who are pretty unhappy because medical professionals chose the wrong intervention in childhood. Only adults of sufficient mental faculties who are under the care of a doctor should be making these tradeoffs. That's how trans worked for ~70 years before recent political activism forced the medical industry to loosen its standards.
Imagine trying to make the same argument about forcing cis women to go through male liberty to "understand men better". It's ridiculous.
Further, studies show that the main predictor of bone structure is whether you started HRT before or after the beginning of puberty, and that outcomes get worse the more it progresses. At 18 you still get some change, but you really need to either block puberty or start HRT before it for optimal outcomes.
And if you don't want to give HRT to trans children, at least get them on puberty blockers. There's pretty much zero evidence suggesting they do anything worse than temporary and reversible reductions in bone density.