Why don't defensive lineman line up backwards and backpedal rush the quarterback?

These defensive lineman would be completely unblockable due to the block in the back rule as they are only showing their back to the o-line. Because of this they would have a free rush on the quarterback and the o-line could only block them towards the quarterback as that is where their chests are facing. This seems pretty foolproof to me and I don't understand why teams have never tried this.