It's mostly because they prefer the term that reaffirms their male identities, the term Tomgirl doesn't really, it implies that they are actually girls which for most femboys isn't true.
Yeah English language is weird, I feel like the term Tomgirl would make more sense for girls that like to be masculine but for some reason it isn't like that 🤷.
I do know that many femboys I have met say that the term tomgirl feels demeaning and also in recent times with egg culture and the idea that femboys are just trans girls in-denial (a really toxic and dismissive mindset) it can feel quite invalidating to many as it implies they aren't really male despite identifying as male. This same thing doesn't happen as much on the other side with tomboys, because gender stereotypes are a double standard, which seems to be why it's more accepted. Misogyny probably also plays a big part in the reasoning behind the naming scheme as well.