Why do pre/pros cost so darned much when receivers can do it all for so much less?
Why? Because separates do it better, period. Most people are happy with an all-in-one unit, and that's just fine. But you won't find an integrated reciever that pushes 500 watts (I'm talking M80 4 ohm units of course) per channel... even 300 watts per channel is a challenge to find under $5000 bucks. But you can find many amps that push between 300 - 500 watts per channel for around a $1000 dollars or less (you can also spend $10,000 bucks on an amp like that... it's up to you).
I'm not saying everyone should go with separates. There are advantages with both. If you find your all-in-one reciever does the job for you then don't sweat it. But the day you buy an external amplifier and listen to your system with it (loud), you will understand.