Players from a position going early or not =/= depth or lack thereof at said position
If there's no depth at a position, but a very good player on the board, that player is going to go early, and teams are going to clamor over one another to try and get him.
Look what happened with the QBs. There were honestly no game changing QBs, but the 3 top rated QBs all went via pretty crazy trades, because after them the drop is pretty big.
Nobody is taking RBs as often because they'll be there in the 2nd, 3rd, etc.