They don't do it for a long time, because that means they will have to KILL support for TVs, Xbox and laptops that already have 4k support. They just use hardware deciding (because of DRM).
Not kill, will just reduce the resolution for them and limit them to H.264 which they support. That would be an incentive to get a device which supports AV1 sooner if someone really needs that high resolution. I'm sure all AV1 backers will do it at some point.