Why are some cab interiors so hard to work with inside the cab?

Well,I often use the reskins of the HP trainz GP38-2's available at NARM driving them from inside the cab moving the controls with the mouse but one thing I find with the cabs and other locomotive cabs is that some of the controls require fiddling with them just to get them in the position that you want it to be in,some of the levers or switches it seems when you pull them back with the mouse that the controls go the opposite direction that you tried to make them go,I am wondering why some cabs are so difficult to use from the inside,I do find it inconvenient to have to fiddle with the controls...

I do plan on getting the GP38-2's at HP Trainz soon to...

You mean that you have to change cam views inside the cab ?

This has to do with the placement of the grab-points on the model compared to their location on the images of the interior.

Sometimes it's easier to use the keyboard shortcuts for the throttle and other common controls than it is to grab stuff with the mouse with the only exception being the locomotive start up if they have that in the locomotives.

I agree with the OP. The movement of the controls with a mouse seems counterintuitive to me. You could try adding

"invert 1"

to one of the controls in the interior config and see if that makes a difference. That tag needs to be set for each control of interest. The default is 0 or "off".