Because there are some like me who watch tv and are more than happy with 2.1 and the (admittedly dubious for some) benefits of an HDMI connection. HDMI use doesn’t equate to 5.1 use.Depends upon what you are after... To me this is a AMP that is focused on stereo output, over surround. Why have an HDMI ARC input when the best you can get out of your sound is 2.1?