Well mostly because it's not really a myth. The only guy who's arguably "not better" than Roger right now is Andy. I agree with the Nadal part (last paragraph), and the fact that the H2H doesn't make him terrible, but your two claims are highly debatable. Even if Federer is "on", Djokovic is probably better on HC if he's "on" these days, and Murray is probably better on grass right now even though historically neither one of them are close to Federer's overall achievements on the respective surfaces.
I still think he's favoured slightly against guys like Berdych, Wawrinka, Tsonga and Delpo, and everybody else, but it's much closer than it used to be and he needs to play well to beat them.
Federer will still have a chance against anyone he plays, contrary to what some believe, but he is not the favourite against Djokovic or Nadal anymore.
I understand your frustration with the inaccuracy of some views like Federer being "terrible" though, and I think that's mostly where your frustration is directed. It is a gross exaggeration, but as I said, I don't think Federer is the favourite in every match he plays anymore, particularly against Djokovic and Nadal.