The answer might be as simple as: maybe because there is more to being transgender.
However, Anne Lawrence, a proponent of Autogynephilia in her 2007 essay titled "Becoming What We Love" tries very hard to defend her case:
“Blanchard (1991) described this phenomenon in nonhomosexual MtF transsexuals: “In later years, however, autogynephilic sexual arousal may diminish or disappear, while the transsexual wish remains or grows even stronger. . . . It is therefore feasible that the continuing desire to have a female body, after the disappearance of sexual [i.e.,erotic] response to that thought, has some analog in the permanent love-bond that may remain between two people after their initial strong sexual attraction has largely disappeared” (p. 248). This insight plausibly explains the reports by some non-homosexual MtF transsexuals that they no longer experience sexual arousal to the idea or reality of being female but still feel a comforting “nonsexual” (i.e., non-erotic) affectional bond to the idea or reality of being female and living as women. If these transsexuals were to misunderstand autogynephilia as a purely erotic phenomenon, they might erroneously conclude that their continuing attraction to being female had nothing to do with autogynephilia”
The first error is to presume there is such a thing as AGP and the term is used here as if it is a given without providing proof. The second is the attempt to try and remain within the AGP model by associating the declining sexual component with what happens to romantic love after many years. To which I would respond: make up your mind, your impetus is either based on an erotic phenomenon or it isn’t. But they try and make AGP malleable enough so we can remain firmly rooted in sexual aberrance as the cause for thinking we are transgender; nice try but it's not so simple.
This is the a bait and switch game of inventing a condition which drives transitions instead of pre-existing gender dysphoria driving identity which then causes the post pubescent arousal. Anyway it's all pure conjecture because no one has any proof and none can be offered.
The first hurdle is you need to conclusively prove that AGP exists which is impossible. But by talking about 3 year olds with erections and losing eroticism with age maybe they hope no one will notice that it's all a shell game.