Over time, due to increasing alienation under capitalism, more men are seeing women as these inaccessible alien beings that they desperately want so they can prove to the world/themselves how manly they are, but can never have for one reason or another.
So to them, femboys are the next best thing because they're feminine like women, but are men, so therefore more "accessible".
Countertheory: non-binary identities and expressions are increasingly more openly publicly visible and seen as a valid expression, and so their popularity among otherwise cis/het people go up because it's just all a numbers game about who is around you and available.
Yeah I agree the gender line is being blurred and young people are more accepting of that, thus stocks go up for femboy demand.
Edit: But I also agree with OP somewhat. I think the younger generations struggle with anxiety and social interaction and that just makes everything worse.
I don't think the gender line is being blurred at all, people are just more apt to call themselves "non-binary".
If you look at teen culture going back decades, since it began, there's always been an element of "gender bending", or claims to that effect. People mocked The Beetles for being girly! In the 80s, the coolest, most popular and "alpha" rock stars wore makeup, had long hair and dressed feminine. Plenty of goth/punk girls rocked a look that almost certainly would be called "non-binary" today. Really what's happened is that people are quicker to put a label on identities that have always been around.
"Femboys" are also literally an ancient tradition going back to Greece, though they used to mean "boys" more literally.
As a nonbinary person, there's something uncomfortable about your comment that I can't quite put my finger on. Maybe as an inroad I'll quote this sentence:
Now the issue I have with the above-quoted sentence is that nonbinary isn't a "look", it's a gender identity (or in my case, since I'm agender, the lack thereof). And I think maybe that's my issue with your comment. You seem to be saying that people are choosing to say they're nonbinary when really they're just gender-nonconforming. And I think that erases the fact that nonbinary people actually exist. Are we getting more recognition recently? Certainly. Are more people realizing that they are nonbinary because we're getting more recognition? Absolutely. But is everyone who is gender-nonconforming just "deciding" to be nonbinary? No, I don't believe they are.
Also, I binge read the book Beyond Pink and Blue (the one !transenby_liberation@hexbear.net is doing a reading group on recently) and it's caused me to think more about gender than I have since I was questioning. The author, Leslie Feinberg, is pretty clearly nonbinary, but for hir it's pretty clear that gender, gender roles, and gender expression are very much tied up together, whereas the modern conception is that they're distinct things that should be pried as widely apart as possible. I used to believe they were distinct things, and I still sort of do, but reading that book has made me rethink whether you really can separate gender, gender roles, and gender expression in any meaningful way in the real world. It's been a little wild.
So this has all been to say gender is a wonky, confusing mess, but nonbinary people do exist, we always have, it's just that now more people have heard of us and so can maybe put words to their feelings, which is a very liberating thing to do, let me tell you.