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.