Because vanilla pi doesn't do a lot of important things out of the box, and they didn't want to open the "which extensions should we add" can of worms.

if anything, using oh-my-pi opened that can more than using pi (or better: both!) would have

Am I wrong in saying that the interfaces presented to the model in OMP versus plain old Pi are identical?

OMP has a lot of candy that raises token cost compared to vanilla pi

it's less about the interfaces and more that it feels disingenuous to compare token usage to a pi setup that injects a bunch of stuff into context rather than the base version. it makes me think they tested it with pi, found they couldnt meaningfully beat its token usage + pass rate, and omitted the comparison. i'd love to be proven wrong, but this seems like the easy explanation