The martini may be simple, but it is not easy to make an excellent one. It's a very solid test of a bartender's skill because, unlike many drinks, ingredients alone cannot carry the cocktail. A piña colada for example, is mostly about ingredients (are you using a good coconut cream? fresh pineapple?) For the martini the chilling and dilution need to be just right. This tests the bartender's most important skill: mixing. Proper mixing of the beverage is ultimately what makes a martini.
In addition, I've had a shocking number of awful martinis served to me.