1. Sub placement. Most people don't even have enough room for the best sub placement. They pretty much stick the subs where they fit. Another factor is aesthetic. Most people aren't willing to place the subs just anywhere. Most people will be place the subs on either side of the front left & right speakers anyway.
2. I don't know the actual SPL/FR. But subjectively, the bass output is a lot for many people with rooms no bigger than 15x18x10.
