In part, it comes down to the same reason that I wouldn’t rewire my own house. I could, but it makes more sense for me to spend my time (a very limited resource) doing what I do best (and that isn’t electrical work) and let those that do that thing best, do their thing. The fancy word for that is comparative advantage (check it out, it even has math to back it up). The idea is that our whole economic system works most efficiently if each party does what they do best and outsources the rest.