Over the years many people who mapped merely as a hobby or for fun have been hired by large and notable game companies to help on new titles. Also, the time which has passed has also allowed some mappers to gain some recognition within the Quake communities. You'll hear names such as Neil Manke, ztn, Dave Waters, SPoG, Wiebo de Wit and others. The communities have come to know that when this name is attached to a map, the map is good. There is little to no doubt that you'll have a near classic on your hands.
Maps also prolong the life of a game.
Therefore, when a game company either refuses or merely shuffles their feet on producing a set of tools for the likes of us to make our own maps, it ultimately hurts the game company that made the game. You can still find sites devoted to Quake mapping, a game which debuted 4 years ago. I'm sure if you look hard enough you can still find authors cranking out WADs for Doom. WADs are maps for Doom and Doom 2. So, you've found your curiosity piqued.