Nobody thinks that selfish leads to "better" outcome.
They believe that if everyone acts selfishly, the outcome is still pretty darn good (but of course could be improved if nobody committed crimes or something)
Again, saying that "the outcome is pretty darn good, therefore yay capitalism" is missing the crucial question - when do the good parts happen, when do the bad parts happen, and how to preserve the former while getting rid of the latter.
Ideologies are mind poison.