Think of the dictionary as less of a Social Register for words and more like a word general store. I am the manager of the word general store. Do I stock only words in my size? Only in the flavors I like? Only the words I wish people would use? No — I provide a wide selection of words for the use of all my customers. And because my customers are such a wide group (basically, all adult readers and writers) I have to make sure to include the words that will serve their needs.
So how does she decide which words to include in the inventory, given the limited shelf space? "Basically, we check to see if people are using it."
This, then is lexicography: checking to see which words people are using, how they're using them, and presenting the information so that other people can use the words in a more-or-less agreed-upon way.
Didn't I just say her job was cooler than mine?