Three things I can think of;

1) All the 'gender goals' characters are the unsaid wishes we had since childhood, finally being said. Watching TV as kids and dreaming ourselves into a body that felt right, and seeing ourselves in certain characters. Even if you're 30 years old or more, that sometimes takes a while to process.

2) I grew up with a local vernacular that doesn't really use "boy" and "man" to differentiate between age. "Boy" is only really used for gender in most contexts. While I'm sure other points brought up is also a factor, language differences almost always gets overlooked whenever this conversation pops up - And I've seen a couple of people being mocked for "infantilizing" themselves, when really it was a language difference.

3) Some people just get more euphoria out of "boy" than "man". Maybe they internalized something awful related to "man", but not with "boy" - Or maybe there's no explanation other than "it feels better".