You say that, but you haven't explained how it is a liability, and I consider it debatable. In practice, there is a near-zero risk to a large firm in a statistical sense if the firm caps the max amount held per user to say $1000 or whatever the acceptable limit is. It's not as if millions of users are suddenly going to exercise their balances all at once unless the firm is going out of business anyway.
Cloud computing firms even allow me to keep tens of thousands of dollars in my account balance, with no expiration, which helps me ensure that my cloud services will keep running for years if not decades. There is just no strong case for OpenAI expiring unused balances after one year. In my prior comment, I already named various large firms that NEVER expire gift card balances.
The reputation loss from an expired balance is vastly more consequential. Thinking solely from an accounting pov is absurd.