Why not isn't the appropriate question. Why? You are making a claim that all LGBTs should agree about the appropriateness of transgender surgeries. Why should they? Just because you want them to isn't a valid reason. That's especially true considering how you are arguing about wants and needs in regards to children in your other thread.
You may think that creating standards of agreement within a particular group upon your whim is somehow a reasonable stance. I don't think you'll find much agreement with it.
Because they pitch themselves as the same cultural movement that just hoodwinked the US Supreme Court by self-assigned assumed identity/"innateness" claimed for so-called transgenders and got the Court to circumvent the separation of powers (by adding for just their favorites, a new protected class to the Constitution based for the first time on odd behaviors) in order to escape majority regulation of a repugnant minority behavior.
LGBTs the "LG" part just pitched cohesiveness "gay gay gay" as a binding agent to gain a legal advantage as a separate class of people. My contention is that if you have a class of people based on behaviors, you'd better well have a clear definition of what those behaviors are and that all the people in that class agree to/adhere to them or else...you don't have a class...you have a bunch of individuals falsely using a label that does not apply in order to gain legal perks.
As I already pointed out, there is no universal agreement on other protected classes. Why should LGBTs be different? Hell, I've heard plenty of people say Catholics aren't Christians. Should Christianity no longer constitute a religion for purposes of constitutional protection?