I’m sure it’s just a cost thing… if you look at the pricing for the realtime API it’s $0.06 per minute in and $0.24 per minute out. That’s about $15 an hour. If you want an unlimited account it’s because you want to use it a ton and they don’t know how to measure that. The extreme might be you’re wanting 10 hours of generated audio per day which would be $150/day or $4,500/month.
I have a ChatGPT+ account and a Tier 5 API account. I make heavy use of both and probably spend 5 - 6 hours a day waiting on a model call to complete. Even with my ChatGPT+ account my API bill typically runs around $300/mo. When I was testing the realtime API it jumped to over $800. Even with text completions I had a day last week that I spent about 15 hours running benchmarks and it cost me $280 for that one day. This stuff is super expensive and I doubt we see truly unlimited anything anytime soon.