For me, it's because I can. When I find a job I do often, I add it to the config, when I find a tool that makes a job easier I pick it up. Bare, freshly installed vim is quite useable, but is missing the affordances of my personally customized environment.
It's kind of like the difference between being able to cook and walking into a strange kitchen and immediately knowing where everything is. In my home I'm comfortable, I know that the vegetable peeler is in a different drawer than the steak knives. In a friend's home, I'm just going to have to ask where something is every few minutes if I want to get work done.