Really good DAC chips are $1-2 parts, and fully burdened with support circuitry they might add $10 to the cost of the AVR. I just picked up a Fiio D07 coax/optical DAC that also handles a laundry list of modern compressed formats for under $50. But most of the money was in the support circuitry and the nifty little case, power supply, cables etc. that are already present in abundance in an AVR.
It is possible that the product planners are fretting over adding $10 to the price of the AVR on the grounds that few of the people who buy AVRs actually run multiple zones.