I know why. Your model is way bigger than what you’re displaying at your viewport.
I get that in all my projects and it happens even if they are small.
The only workaround I’ve found is to have your model displayed in square shape viewports. Sometimes it also works with rectangles of 2:1 ratio.
However if you are zooming in as far as 1:1 scale in a bigger model, you might need to make the viewports way bigger and then clip mask them, as neither square viewports work.
NOTE: Actually, clipmasking viewports is usually faster to set them up than scaling and re render them. So you might consider clip masking as a usual workflow.
Take a look here: