Because the alternative is an undifferentiated duplication of the same two or three builds endlessly. Here's an analogy I like to use to explain the concept ludologically. Another analogy is "If you like sushi, and you like chocolate, and you like beer, and you like pizza, you don't get the best food by putting all those things in a blender and drinking what comes out."
Hell, isn't that why they put ATs into the game in the first place? I always heard that back in the game's Alpha development, chargen was all open, and eventually the One True Build emerged and it was all anyone played (funnily enough, my recollection of the OTB is that it was what we're now calling the Sentinel).