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.