Who is they? The alt right? Can you point me to their official website? What you have is a bunch of kids on Reddit and 4chan making memes and dissing RINOs and the establishment and then some small percentage of them goes racist and now the Alt-right is being cynically used by synecdoche to use that one part as if it were the whole.
How big of an audience does some fool like Spencer command? Who put him in charge of the alt right? What the media has done, quite successfully, is use a catch all term to specifically mean white supremacist, so that a very diverse and unorganized group of people with many different agendas can all be painted as a white supremacist, regardless of their views on race.
One of the values of such labeling, is that it ultimately separates out the racists from everyone else formerly part of the much more loosely defined alt right.
Alt left is also a catch all term for half organized groups of professional protesters, revolutionaries, anarchists, etc with sometimes wildly different and even incompatible agenda. Those folks exist, so if you don't like "alt-let and you don't think they are mainstream leftists, you'll need another term in any event.