When you focus using the focus points visible in the optical viewfinder, you are using phase-detect autofocus (PDAF). The sensors for are located on a secondary sensor that is not part of the imaging sensor, which receives light reflected off the mirror while it is down. These focusing points are fixed in location and cannot be moved (although they are typically a bit bigger than what's actually displayed in your viewfinder).

On the other hand, live view uses contrast detection autofocus (CDAF). This uses the actual imaging sensor, and works by analyzing the image that the sensor itself is receiving with the mirror up. Because it's using the imaging sensor itself, rather than a bunch of secondary sensors in a fixed grid, you have more flexibility in choosing your focus points.

This cannot really be changed, other than by switching to a camera body that has a denser pattern of PDAF sensors. Although, the D850, with 153 PDAF sensors (some with varying characteristics), you're already towards the top of the pile in that respect - some older PDAF sensors only had e.g. 9 or 15 elements.