It is actually harder to produce in 4K/8K due to more details that need to be drawn to not make it looked too empty and need to make sure the lines are not too thick (e.g. by using a larger paper). TV series are usually drawn on an A4 paper with 1-2 inch margin while a proper theatrical releases are drawn on a B4 paper.

Another factor, I believe, is the know-how. In my opinion, despite anime being broadcasted in 16:9 for so long, it is only in recent years where the extra width are put in a good use during layouting.