If there was a design tradeoff that allowed to somehow massively sacrifice efficiency for very low low specific mass and extreme heat tolerance, we might see the discs replaced with in-wheel "harvesting brakes" in addition to inboard high efficiency cruise motors.
But #1 I doubt that kind of tradeoff exists, at least not in sufficient magnitude, and #2 it would probably be quite uneconomic to exploit, lots of lower hanging fruits to pick first.
Also, a larger rotor weighs more than a smaller one.
Combined, a large wheel and tire will require more energy to move and to stop, and will add as 75 pounds or more to the weight of the vehicle. This has negative impact on handling and fuel economy.
The only reason why the manufacturers add a large wheel option is because people want them.
Larger wheel vs larger rim is not the same thing. A larger wheel has a larger weight but smaller rolling resistance. A larger rim on the same sized wheel has better handling characteristics and more room for larger discs, whether it is on a performance vehicle or a regular one doesn't really matter, plenty of mid-sized sedans of today are yesterdays performance vehicles.
There are outside exceptions (any SUV, ridiculously sized wheels on many American cars and people driving trucks when a normal passenger vehicle would be more appropriate). Those do have worse handling and fuel economy, but they're the exception, not the rule.
[edit]
Disc brakes add about 9.5kg on the front[2] (maybe half that on the rear?), so a single disc brake rotor weighs more than the entire weight of all 4 motors.
Obviously this is using Magnax's numbers for very large motors and assuming they scale down, which is silly, but clearly there is some weight at which it would make sense, particularly for a non-high-performance vehicle (1 motor per wheel has lots of advantages for handling, but AFAICT this is not unique to in-wheel motors).
1: https://www.tirerack.com/wheels/tech/techpage.jsp?techid=108