OpenAI does not make money from math papers LOL they just release them for the sake of community. (Because sitting on those results would be considered worse.)

You don't seem to understand how the company evaluation and perception works. I would suggest reading on that.

Solving a well-known open problem like N-S "millennium" problem has some marketing value, as this can be referenced in a press release, etc.

OTOH a marginal value of a single paper in a paper dump is basically zero. Nobody gives a flying fuck if they made 100 or 200 papers. It won't give them even a single customer. If they released, say, 5 paper per month it would generate a buzz but not as much backlash from mathematicians.