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.
