The US does! The majority of internet traffic in the USA (around 55%) is via IPv6. This was driven by two things:

1. Comcast was the first major ISP worldwide to roll out IPv6 to customers, 15 years ago in 2011. They finished the rollout in 2014, and 70% of their customers were using IPv6 within a year.
2. Mobile carriers use IPv6 a lot. T-Mobile's network is fully IPv6-only, using 464XLAT to connect to legacy IPv4-only servers. No CGNAT. They've done several presentations about it. The other carriers aren't too far behind.

The fact that IPv6 usage isn't higher is mainly limited by servers not having it enabled, not users. IPv6 usage spikes on weekends and after business hours because there's a lot of enterprise systems that people use at work, that only use IPv4.

It's frustrating that other countries adopted CGNAT instead of switching to IPv6 like the US mobile carriers did. Argentina has less than 30% IPv6 adoption for example.