Simple answer: Because they are designed to work that way. Mostly due to the increased use of subwoofers the speakers no longer need the bass reinforcement of a near wall or near corner set-up.
Why does pulling out main speakers from wall improve sound?
Ask my dealer this question and he was stumped. He said it's a good idea but couldn't say why. I see speakers pulled out eight or more feet from the wall in very nice systems.
The drivers are facing forward, and when there are no ports in the back of the speaker so why would it matter?