If you are using a computer as a professional tool why wouldn't you put in the effort to customize it? It's mostly a one-time cost and lasts a lifetime. It's like using a stock car as a race car.

Because it's not really a one-time cost. It's a cost for every computer that you sit down at. If you're only ever at one computer, that's great, but in practical terms, it's more likely that you'll have your home computer (possibly more than one), your work computer (possibly more than one) and your team mates' computers. And if everyone's computer is a little different in some subtle way, that's a big problem. If the default settings, on the other hand are good enough that few if any tweaks are necessary, that means that the experience of moving between machines is less painful.