Theoretically, we could try to calculate it based on the geometry of the shot, but we'd have to make a number of assumptions in the math that would make the displayed chance to hit inaccurate, and we'd still have players asking "why didn't my 100% hit?" - it's no better off than we are now.

Another option is that we could simulate a large number of theoretical shots based on the exact situation and display a percent of how many times those theoretical shots hit, but in order for that to be reliable the number of simulated shots would significantly slow down the game whenever you're aiming.

In either case it's not helpful to show "true percent chance to hit" and not worth discussing the fact that it's theoretically possible.