Ravi,
It's limitations on the size of an A/V receiver's power supply as well as cooling/heat-sinking issues when you cram seven amplifiers into a receiver chassis along with all the other video and audio switching circuitry. An A/V receiver manufacturer has to meet cost targets vs. physical size limitations--the thing has to be able to fit on a standard shelf! As it is, a few flagship A/V receivers (Denon, H/K) have become almost unmanageably large. Several of these will produce greater output (Denon and the H/K 7300). I would have gotten the earlier model, the H/K 7200, but it wouldn't fit onto my equipment shelf.
A designer of a basic power amp has far more room on the chassis for robust power supply, big capacitors and lots of heat-sinks, even with a multi-channel basic power amp. Since there are no controls, you can stick the power amp out of the way so physical size isn't a big deal.
With advances in digital amplifier designs (on-going), which run cool and do not require huge heats sinks, some of these limitations of analog power amps in receivers may be overcome. In Axiom's EP600 and EP500 subs, we use a 500-watt digital amplifier that runs cool with a massive analog power supply so that there will be headroom.
But there are shielding issues and many others that have to be addressed if digital amps are to be used successfully in an A/V receiver.
Regards,
