yes it is back because black takes on c4! after Nc3 Nf6 Nf3 a6(or many other moves)Bf4 dxc4 and black is already better

I don't really know but my guess is that it sort of losses time, because when white sacs a pawn in the opening his moves have to be very accurate and do something in order to get anvantege, while B on f4 just doesn't look that great, because the fight will have nothing to do with e5, d6 and b7 squares after black takes on c4. Plus often black plays here Bb4 after some time and with white's B on f4 white's knight will have to stay on c3 because of the pin.

So in general this black-square bishop is not so important because it can't contribute much to the fight on the queenside that is about to come after 3.Bf4, so why to move it already in the 3rd move? That's my opinion, I'm not sure if it's correct, but practically in all 3.Bf4 continuations I've seen this beautifully centralized bishop doesn't play any significant role in this opening, but it costs one tempo.

quite simply, knights before bishops.

Bf4 can be made non-useful in some lines.  Knight moves can't.