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

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.