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.