Because you can't really draw all that well.
This is a small selection of the hundreds of maps I've illustrated for official video game strategy guides over the past few years. This gallery includes in order (3 of each): Call of Duty - Black Ops. Dishonored. The Last of Us. Tomb Raider. Call of Duty - Special Ops. Halo5. These are the final submitted concepts before they were integrated into the book designs by other talented artists at Brady Games and Prima Games.