I'm pretty sure there is plenty of 444 content. Movies are likely filmed/edited at 444, they are provided to theaters at 444, but they are generally only made available to consumers at 420.
I believe the reason is that most hardware decoders sold to consumers do not support anything above 420. As Blu-rays and such are all currently 420, the chip makers don't have much incentive to support 444. It's a shame because your TV fully supports 444, and the difference can be huge on some content.
The good news is that we don't need to change the H.264 standard for this.
Supposing that:
- most of the already deployed embedded H.264 decoders support 4:4:4 profiles
- non-chroma-downsampled input content is available
Then, it's only a matter of changing encoder implementations to do exactly as you say.