Opposite end of the aisle here, but I hope my comment is welcome. I think a lot of this stems from the bioessentially prescriptivist attitudes a lot of people have. It's for similar reasons that trans women are often framed as predators in women's spaces that trans men are framed as "smol beans" to protect, or misguided by the Trans Agenda™. It's an assigning of hyperagengy to AMABs and hypoagency to AFABs, and it's really gross.

The most "fun" part I'm guessing is when you call it out as a reactionary attitude and cue the shock and horror to the thought, that maybe unsolicited infantilization doesn't paint transmascs in the positive light they think it does.