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.
Your comment just made me realize something; even in time periods where men were more socially expected to "grow up" and act like an adult instead of in this state of arrested development and so on, there was still never fully a time when the role models put forth of 'being a man' carried the expectation that they fully learned to take care of themselves and fully handle their own chores and affairs independently. 1950s "head of the household" types who never learned to clean their own homes, do their own laundry, cook their own meals, but also 19th century 'confirmed bachelors' who tended to either live in a house with a female housekeeper or in a boarding house with a woman handling those things (according to their level of wealth). Maybe you can look to grizzled trappers and literal hermits, but even trappers heading out into the wilds often got a wife (one way or another) to do a number of those things. Soldiers have their meals served to them. Monks and priests often still depend on nuns to clean their homes and have someone dedicated to cook for them. The role models of male adulthood almost always still come with an invisible woman taking care of them, doing the things they don't want to do, the daily drudgery, and men currently have less expectation of being able to access women's labor like that than ever before, and rather than putting in the effort to learn how to fully take care of themselves and learn how to do 'women's work,' many are just becoming NEETs and refusing to grow up at all. But all of their role models for male adulthood, "being a man," still depend on others taking care of them, particularly women, whether invisibly or not.
As a society do we need PSAs directed at adolescent boys featuring manly men doing their laundry, paying their bills, cooking daily meals, cleaning their homes, going to work?
Yes, in short.
Dunno, I'm a bit burned out right now with exactly this issue at home and feeling pretty salty about it. But in short, all that boring day-to-day stuff has always been "women's work" and is expected to just happen somehow (what, done by fairies?), even now by otherwise enlightened (ha ha) men who grew up with fathers who pulled some of their weight.