Because nobody wants to learn about the plights of being a man in the modern world men are leaser beings in modern society I remember seeing a video where a bunch of female to male trans were talking about how horrible it was to be a man and how nobody told them it as hard as it is that's why nobody writes about it
I don't think that people avoid writing FtM GBs because they're intimidated by men's issues, because most people don't think that men have issues, to begin with. Sadly, many of men's real issues (toxic masculinity, abuse, etc.) are swept under the rug by women and men alike because it's an uncomfortable topic and seen as a secondary concern. Plus, many GBs aren't centered around the struggles of being the opposite gender because the main appeal in GBs doesn't lie in the struggle. It lies in the fantasy. It lies in the prospect that your life would be better or more interesting as the opposite sex. Realism is often forgone in many GBs.
But I do think the difficulties that men face would serve as great material to write a GB on. To bring these things to light in a GB would actually be quite novel and interesting. Reminds me of
Mimic Royal Princess, an MtF GB where the MC pretends to be a girl in a society where men are oppressed. It's set in a fantasy world so it doesn't reflect many modern-day struggles, but it's good nonetheless.
I think that we should probably consider the huge popularity of slash (fan)fiction and yaoi. At least while discussing the cis authors writing. Like mentioned before a lot of cis male authors will write mtf gender bender stories because A: they suck at writing female characters, and B: it's statistically a pain in the ass too ask men to empathize/read stories with female mc's so you can grab a male audience if the mc is "male".
But cis women authors have no compunction about writing male characters, heck 44% of stories on AO3 are m/m ships
Source and the vast majority of it is written by cis women authors. And a vast majority of yaoi outta written by/for women as well.
I'm pretty sure this has a big impact on at least the cis authors side of the equation. But I don't really know enough to really expand on it.
And yes, this is very, very true. I didn't really consider it, but it certainly makes sense. I think in general most cis-women (if that's the audience we're assuming here) don't fantasize about becoming a girl one day nor are they particularly fascinated by men (especially men's bodies) the way cis-men are. This is likely a societal/cultural thing. Throughout history, it has always been men who held the center stage of media whilst women's lives were often shrouded in mystique. Views on masculinity/femininity have also made it more taboo for a man to act like a woman than it has for a woman to act like a man.
With most stories featuring a male lead, I think many cis-females, as
@Sylvie stated, are perfectly content reading novels with a male MC and not a genderbent one because (1) they're accustomed to it, and (2) they don't have that curiosity of what it's like to be a man because everyone kinda already "knows" from all the male-oriented media out there.
But then again, all of this is just conjecture. I definitely agree to some extent that BL is the cis woman's gender bender though.