The reason why photo taken on camera is much better than video is because, it doesn’t require as much processing and much of the processing goes into perfecting a single frame of photo thus improving the quality of photo greatly.
A video is like taking a bunch of photos in a very short period of time (for example 60 fps meaning 60 photos in 1 second). This require a very high processing rate of images taken within that short period.
hope that answers your question:)