Envy and hate are two reasons why.
I was more popular in high school that I thought I was. Several girls had crushes on me. I didn't have a clue (as I written before, I am not an "Alpha." In high school I looked like Garth Algar, only with better hair).
As an aside, I now look like Walter White, including the shaved head and goatee.
Again, in college I was more popular than I thought. Three girls admitted they had crushes on me at the same time. Again, I didn't have a clue, although one told me later, "It was obvious." Obvious to who? Me? No, it wasn't.
In college I encountered something I had never encountered before. Envy. I was such a low envy person I didn't recognize it.
For about a year I lived in a co-ed house, meaning two girls and about seven guys. Both girls ended up in my room, but not any of the other guys. Three came home with me from various places. Five in a year.
Of the other guys, not one ever had a date, not one had a woman spend the night. The attitude of some of them was decidedly hostile to me. I didn't get it.
Years later I realized it was envy, which creates backstabbing and the attempts to put the envied down. I realized later they were thinking, "Why that bastard and not me?" Because I'm better than you, that's why.
Let's put it this way: all these "Alphas" and "Sigmas" don't talk about others envying them, sabotaging them, denigrating them. Yet if they were these things, they would be very familiar with less successful guys envying and attacking them. But they never write about it. And that is one reason I think the "leaders" of the Manosphere (they are only leaders because their followers made them that way) are not telling the truth. Perhaps because they don't know as much as they pretend? Perhaps they not the Alphas they pretend to be, are not as popular and successful as they lead their readers to believe?
One other thing. I sort of fit the "Sigma." And I've noticed something about that which I have written about before. When you tend to be funny as I am (I recently told a woman, "When women get involved with me their lives always get better" which caused her to say laughingly, "God, you're arrogant") many women either love you or hate you. This is something never written about in the Manosphere: that some women can't stand "Alphas" and "Sigmas." And there is nothing about this. It's all about lifting weights and approaching a thousand women to get one "bang" - overwhelmingly fantasies. There is nothing about the Dark Side.
Not every woman is going to say, "Ohmigod! George Clooney! An Alpha! Superior Alpha genes! Resources! My hypergamy is tingling!" I know women who don't find him attractive, just the way when I was in my 20s I had women tell me they didn't find Robert Redford attractive.
I have been the object of a noticeable amount of hate and envy in my life. It is not a pleasant thing, and real "Alphas" and "Sigmas" should have experienced the same things.
So when these more influential bloggers starting writing about how much they suffer from the envy of others, and how some women are not attracted at all to "Alphas" and "Sigmas" (and for that matter, don't despise "Betas") then the day will come when I take these writers a lot of seriously than I do now.