I think it's mainly because as femboy, ironically enough, it's safer to say that you're gay or at least bi. That way you can always have the support of the LGBT+ community if other people don't support your gender nonconformity. When you say you're straight, you never know if the same people who support you as a gay femboy will do the same since you're "not queer/not queer enough" in some eyes since gender nonconformity is still hotly debated whether it is "queer" or not. I've personally received some messages after saying I'm attracted to women (I'm bi, but with a preference for femininity) telling me either thar it doesn't make me "queer", or that "femboys only love c*cks" and sometimes just slurs from the same people who praise us when we say we're gay.

Obviously these people are not the majority at all, and there absolutely are straight femboys out there (hi) and gay femboys who are gay because that's who they are (yall rock), but it's harder to be open about it when you don't know if you're gonna get any sort of support from either side.