20 centimeters of air is a LOT more than it sounds like. I personally run a pressure of 14 cmH20 and the airflow needed to maintain this is a LOT higher than that needed for breathing. I run a full face mask and my machine averages flow of around 30 liters a minute. The air needs to be "turned over" constantly since you don't want to be re-breathing stale air.
(As a side note to GP: Reducing/stopping pressure on the exhale is problematic also... for some of us many of our problems are actually on the exhale.
I know basically nothing about how CPAP devices work, but why does it need to move so much air? There are snorkels on the market with little check valves so your exhaled air goes out a different pipe than your inhaled air. They cost maybe $20. I can't imagine that a super-expensive CPAP machine doesn't have a similar pair of valves.
Basically, because you're sleeping, and even a good mask seal will leak a fair bit, and there is also a lot of intentional leakage around the valves because you don't want stale air hanging around, or condensation building up, and you have to make sure that the person can breathe sort-of normally if the power fails or something like that.
As a guess: with a plugged-in device, the manufacturer doesn't care about power consumption, and the thing already needs to be be able to supply enough air for the peak flow while you're inhaling, so it doesn't really matter if it leaks.
If they cared about power consumption, they could presumably make it less leaky and use check valves to prevent issues with stale air or inability to breathe if the fan stops working.
I've used snorkels with separate in and out tubes and check valves to route the air correctly. It solves stale air problems (which are irrelevant for snorkling because the total air volume is much less than the amount of air you breathe in per breath) and causes serious suckage (because you really want your exhaled air to displace water that comes in the intake, and check valves prevent that), but for CPAP it might make sense.
As I suggested, I won't be shocked if it is infeasible in the end, or if it practice it only works on the lower end of the scale, or that it has significant issues with the other considerations you laid out in your deeper reply, such as condensation. I just know that we can't do the math as if we expect this little thing to be doing all the work.
(As a side note to GP: Reducing/stopping pressure on the exhale is problematic also... for some of us many of our problems are actually on the exhale.