This is in part "abuse prevention"[1] and in part marketing. Making customers feel like they're signing up to access state secrets makes the models seem more "special". Sama is well known to use these SV marketing tricks, like invite-only access, waiting lists, etc to psychologically manipulate users into thinking they're begging for entry to an exclusive club instead of just swiping a credit card to access an API.

Google tried this with Google Plus and Google Wave, failed spectacularly, and have ironically stopped with this idiotic "marketing by blocking potential users". I can access Gemini Pro 2.5 without providing a blood sample or signing parchment in triplicate.

[1] Not really though, because a significant percentage of OpenAI's revenue is from spammers and bulk-generation of SOE-optimised garbage. Those are valued customers!