Which Apple TV model? I believe the 1st gen 4K still doesn’t have it at all due to performance considerations. There are many devices that still don’t have it and some may never get it.
Also keep in mind that at 720p the videos can be of limited use. Maybe good enough for your purposes, maybe not. For me the resolution is perfectly adequate for filing bug reports, but too low to incorporate into YouTube content.
Earlier generations of Apple TVs have weaker and lower-performing processors, which is why the feature isn’t available for all models. However, we are still testing and evaluating performance on Apple TV 4k (1st Gen) - I can’t yet confirm though whether it will be broadly released to that model at this time though.