Because the player's computer needs to know where the enemy is in order to render them on the screen, create footstep sounds, calculate shadows, etc. As long as the player has ultimate (root, admin, etc) access on that computer, it will always be possible for a program running with elevated privileges to read that enemy position data from the game's memory and make the required mouse movements to point at it and left-click.
The only way to prevent this is to remove elevated access from the player's computer. This has been done with varying levels of success on consoles, but even then it's only a matter of time.
