No other reason? What about simply fraud protection. The same reason they switched new accounts to be where you have to pay to buy credits first instead of paying at the end of the month. There is a ton of fraud in this industry

The other reason could be the copyright cases they are fighting in court. OAI was ordered to keep all records, including private. Not sure if it was lifted already.

And another could be EU requirements for age verification. AI can produce adult content.

There are may be other reasons, like to prevent using OAI models' output to train competing models.