The reason is simple. It is because of the current architecture of home AV. This is related to the addition of sub outputs, with crossover and the LFE channel.
So, if you don't use a sub, you have limited bass control and quite possibly no LFE if you can't direct the LFE to the mains..
I don't know of a commercial system like mine where the main speakers can accept discrete line and sub outputs and with the BSC signal carried to the bass woofers. I am not aware of any other arrangement like that, but there could be one out there somewhere. You won't see a sub in my AV room, but if your heard it, you would think it was stuffed with them and wonder where they were hidden.