It is simple. If there is a mechanic in the game that allows the player OR THE COMPUTER to simply apply one strategy to completely destroy the other, that is no fun. If I have to ALWAYS take high ground to win every fight or the computer will, that's not fun. Nor is that "realistic". Just because someone has a higher elevation, it doesn't mean they can hit twice as good with a bow as someone on a lower level. A person who can't shoot a bow well won't suddenly become a sniper king just because they are on a hill.
