5 speakers (2x40mm, 30mm, 2x20mm) in each ear draw more power as a group than the single (usually 40mm~60mm) speaker built into most gaming headphones.
A single USB2 port only provides 5V up to 500mA (max), 2.5W (max), apparently not enough.

Proper impedance matching must have been difficult to accomplish unless the DAC outputs somehow plex or distribute frequency weights across the individual speakers in the array. But I suppose software processing fixes things up a lot and waveform distortion/loss would not be particularly critical (very noticeable) on a "32Ω" speaker array anyhow, lol. The Strix 7.1 gets generally positive reviews, so they apparently got it working well enough. I'd be interested in how it "really" sounds (subjectively) when plugged in through an amp.