For me I feel it's the latter, it's just inherently adorable to see a cute girl act like a boy for whatever reason.
Biggest factor is that "compassion" bit I think, the relatability and camaraderie is an attractive prospect, someone you can truly talk to like a friend and roughhouse with rather than wearing kiddy gloves. And the idealized expectation that they'd be chill and have less inherent "drama" than a normie woman.
Plus strong, warm personalities are always desirable. Tomboys seem like they'd be fun to be around and that they would want to go on adventures and push us out of our comfort zones, make us have new experiences.
I'm also into the "corruption" thing and a lot of people seem to be, I guess it does have to do with the natural dominant position of men but overall it's just super cute to watch them be pouty and complain about wearing a dress and it's hard to say why exactly. Where'd we all get this fetish from?