Hi Kaynin, I would agree that if extra HW is required that could be the reason.
I don't think anything extra is required though, an AVR already takes in audio signals from both digital and analog sources and plays both fine through the main zone speakers. It seems odd that the same audio cannot be routed to zone 2 whilst the analog input audio can.
My assumption is that all audio routing to speakers must be done after the DAC is performed, but maybe that is not the case. Perhaps the systems are so simple that they just put a stereo speaker relay on the aggregation point for the analog audio inputs and do nothing to the DAC audio. Thinking about it thats probably the case and as you said it all comes down to cost.