It's worse than that, for partial successes they encourage people to submit the attempt but reserve the right to not pay anything (they may, at their discretion, give a partial reward if they feel like it).
Because it can't and it's a publicity stunt. It achieves three goals:
1) Underscores to the general public that the models are amazingly powerful and if you're not using them, your competitors will out-innovate you,
2) Sends the message to regulators that they don't need to do anything because the companies are diligent to prevent harm,
3) Sends the message to regulators that they sure should be regulating "open-source" models, because these hippies are not doing rigorous safety testing.
Both Anthropic and OpenAI have been playing that game for years.
If it's an existential threat to humanity, and if OpenAI is valued at nearly $1T, why set the bounty at a measly $25k? The going rate for an iPhone zero-day is six to seven figures. Some companies will pay you more than $25k for a website XSS.
Because this is not a serious effort to address a serious risk. It's a PR stunt, the bounty is for a simple jailbreak and not a bioweapon, and they don't necessarily want to spend a lot of money or get people really invested in breaking their safety filters.
They’re probably expecting biological weapons of mass destruction can be created without too much effort, so are curious to see all the nifty ways people can create biological weapons of mass destruction?
Though it could be a Honeypot they are probably hoping to train on all the ways someone might try to do this. Or maybe funds are really low and they need a smoke screen for a really bad actor to go in and try to do it for real.