Why is 2 Channel better than multi-channel?


I hear that the music fidelity of a multi-channel AV Receiver/Integrated amp can never match the sounds produced by a 2 channel system. Can someone clearly explain why this is so?

I'm planning to upgrade my HT system to try and achieve the best of both worlds, I currently have a 3 channel amp driving my SL, SR, C and a 2 channel amp driving my L and R.
I have a Denon 3801 acting as my pre. Is there any Pre/Proc out there that can merge both worlds with out breaking my bank? Looking for recommendations on what my next logical steps should be? Thanks in advance.
springowl
OK, EVERYBODY, IT'S OFFICIAL! Kr4 HAS SHUT THE TREAD DOWN! R.I.P 2 ch v multi thread!!! It's been real...

...we can all go home now and start on the next Oppo thread, or mull over subwoofer placement, which preamp sounds better with what rare tube, or try in vain to research which non-matching other branded center speaker ill-advisedly go with our Class B Stereophile rated dedicated music only loudspeakers!
Mission accomplished soldiers!
Let's just all move on then.

....ok aaaand we're done!
OK, EVERYBODY, IT'S OFFICIAL! ..Kr4 HAS SHUT THE TREAD DOWN!
...we can all go home now and start on the next Oppo thread, or mull over subwoofer placement, which preamp sounds better with what rare tube, or try in vain to research which non-matching other branded center speaker ill-advisedly go with our Class B Stereophile rated dedicated music only loudspeakers!
Mission accomplished soldiers!
Let's just all move on then.

....ok aaaand we're done!