Because gender affirming care isn’t just surgeries. It includes things like hormone replacement therapy and especially puberty blockers. By waiting until after 18, they’ve already gone through most of puberty and its damaging effects to their body. If they still want to transition, then that’s tens of thousands of dollars for surgery that might not have been needed if they were allowed to start on puberty blockers at least.
There’s also the fact that banning gender affirming care is banning life saving healthcare. That is years that a person needs to go without receiving proper treatment. It’s wrong for the same reasons that banning any other medication for anyone under 18 would be wrong.