The issue is that, while x265 is more efficient, it’s not THAT much more efficient until you get to 4k or high bitrates. Encoders using x265 tend to be overly focused on file size, and prioritize it over video quality. And that sort of makes sense - x265 needs a lot more decoding power, and excludes a lot of otherwise capable devices. Why would you do that to only save a small percentage of the space needed?
The issue is that, while x265 is more efficient, it’s not THAT much more efficient until you get to 4k or high bitrates. Encoders using x265 tend to be overly focused on file size, and prioritize it over video quality. And that sort of makes sense - x265 needs a lot more decoding power, and excludes a lot of otherwise capable devices. Why would you do that to only save a small percentage of the space needed?