You only need fairly simple circuitry to accept a standard line input in the processing chain between the Preamp board and the power amp board(s) ... in many cases they already run at close to standard line level... that (in theory) should not be an overly onerous or expensive additional feature!
Agreed, just that in my opinion there isn't much point doing it for the entry to some midrange AVRs that have weak power amp sections such as Marantz's low profile series and Denon's AVRs below the X3800H models. That would be fine for those who don't care about SINAD less than 80 dB. Keep in mind for SINAD 80 dB in the 1 kHz test, within the full audio band you can expect below SINAD 70, or even less (60 dB), that's still acceptable to a ton of users who really don't care about measurements, but for those who care, and know enough to want to use their lower end AV receivers as power amps, they likely want something better. All just our opinions though, obviously highly subjective in nature.
Pre out is different, it is easy for the likes of D+M, Yamaha, Onkyo to standardize as we know for a fact that D+M has been standardizing their preamp/dac section, not 100% but at least keeping the DAC, OPA, volume IC, switches, the same for their whole midrange to flagship models.
So, it is worth equipping them with pre out/preamp mode so users have the option to take advantage of this section by just adding external power amps at can be very affordable now with many class D amps available. Manufacturers may never go for it, because their marketing/sales teams would likely say no, for obvious reasons.
