I wouldn't say all flagships are necessarily minimal, but there is quite a bit of acoustic engineering that goes into it.
Probably two of the main issues with headphone housings are resonance and reflections. Resonance being the effect a vibrating driver unit has on the actual driver housing. Cascading vibrations into the housing can cause negative effects with the sound quality.
Sound is also reflecting off the surfaces in the housing, and techniques such as open back designs and minimalistic enclosures can help reduce the reflections occurring within the headphone enclosure itself.
I do think people underestimate just how much thought and engineering goes into the actual housing. People tend to focus disproportionately on the drivers, but to use a car analogy, an engine will only get you so far, a chassis shaped like a brick wall will be the limiter of your performance as speed increases and wind resistance takes over. Wind tunnel research and testing is incredibly expensive, and yet it is an underappreciated aspect of car design as people generally only focus on the engine.