Because they are pushing a narrative in an effort to get around the 2nd amendment. In addition, they do this every single time an election is coming up.
An AR-15 (and no, the AR does NOT stand for 'Assault Rifle', it stands for Armalite Rifle...Armalite being the company that originally developed it) is barely more than a .22 and has no option for automatic fire. The option to switch between semi-automatice and automatic is what makes and assault weapon an assault weapon, not a pistol grip, black stock, placement of magazine cavity or barrel shroud. FOr further clarification, see the images below.