The cynic in me is wondering whether it's more about how revealing how the sausage is made might bring bad publicity.
Imagine if their target customers, C-suite execs looking to replace workers, knew how unlike "thinking" this process actually was! we can't have that.
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It's to mitigate their competitors ability to run distillation on their models. The only advantage frontier models have is being at the frontier.
There's nothing in the reasoning tokens that'll give bad publicity that the final output already wouldn't do.