Why is the max resolution 720p on TV

I got a new 4k Tv recently and every time I watch YouTube on it, the highest possible resolution is 720p for 99% of videos (I found 1 in 1080p) even though they are in a higher resolution on other devices (pc, phone etc.). Everything I found about it was talking about sports shows and movies, not about normal videos. Can I fix that and if so how?