Possible alternative explanation (not saying this explanation is necessarily wrong): if people have known someone before they were out, and they thought they understood them as a certain gender, and then they come out as trans, they might specifically want to understand transition from them because they're trying to reconcile their previous understanding of them with the new understanding. Whereas long term trans people may have been understood as trans or at least known as trans for decades and so there is no cognitive dissonance to reconcile about them?

For example, Elliot Page was famous before he transitioned, a lot of people were fans of his thinking he was a woman, then when he came out a lot of people wanted to bridge the gap in their minds between "what they thought he was" and "what he actually is". I would say most people who bought his book were fans of his who wanted to understand him better, not people who wanted to exploit a vulnerable person.

Not saying there aren't nefarious reasons too but I think from a celebrity perspective it can come from well meaning people who are positive but are trying to connect the two schemas in their minds of "who you presented as before" and "who you are now" in a coherent way.