I mean, I know why, it’s just so fucking creepy.
I believe there are a few reasons.
Pedophilia. Let’s get the obvious one out of the way.
The belief that women are inherently childlike. Since TIMs rely on stereotypes, a common favorite is the Cute Little Bird-Brain. I’ve said before that TIMs have misogyny towards women, and part of that is the belief that the lowest man is smarter, more successful and more capable than the highest women. And sometimes they state out loud how “easy” it is to be a woman- you don’t have to think about stuff so hard, you can just be girly and breezy and leave the hard work to the men. Because they think being a perpetually-infantile ditz is all part of being a woman, they behave in a childlike way because they think that’s how women are supposed to behave.
Genuinely missing childhood and wishing to relive it. This is somewhat related to the above, but they view being a man as hard: responsibilities, deadlines, having to “act manly.” They want to go back to a phase where they didn’t have to do any of that, but also combine the “it’s so hard to be a grown-up man” gripe with being a woman. It’s the same reason why, despite “being women,” they don’t exactly jump at the chance to vacuum the floors. They view both womanhood and childhood as being inherently easy and free from duties.
Thinking that their interests are inherently childish. This is only tangentially related to being trans. A lot of this stems from the belief among some people that watching animated films, playing video games, putting together Lego sets, etc. is only for kids. Since they enjoy those things, they think they must be children, too. Again, people would be a lot more content with themselves if our society would stop grouping things into “for women” and “for men” and “for adults” and “for kids.” It all stems from a dependence on the belief that there are rigid roles to follow, and failure to do some makes one someone else. In this case, the fact that they’re trans is merely part of their general insecurity with their identity.
I think it's a combination of the first two. They want to embody male sexual fantasies. It's the same sexuality of a gross man, just directed at themselves. They of course will recruit other people to be props in the fantasy, but they want to be the one who full embodies male sexual objectification of women. Think of the man who goes down to the beach to salivate over adolescent girls in bikinis. It's the same sexual fantasy about teenage girls, except the AGP wants to BE the teenage girl, and he wants to find another man with the same disgusting desire for sex with a teenage girl, and act that out on him. When you look at it from the inside, it doesn't make as much sense as when you abstract it. The dumb bimbo woman is a male sexual fantasy. A woman who has no needs, desires or thoughts, so she's completely receptive and passive to male desires. She has no scruples, she's always down for sex, she doesn't resist the male no matter how violent and depraved he is. He has to put in no effort in attracting her or convincing her or soothing her anxieties or answering her criticisms. She's just there to be fucked. The AGP wants to be that, because the AGP wants that.