Because sometimes you are expected to write working code on whiteboard. Once interviewer was typing code I wrote on laptop and interrupting me on errors he had.
I won't switch from Mac anytime soon, but competition is always good. Can't wait to see how Apple will respond to latest Microsoft actions (WSL, Surface Book and Surface Studio).
MBP is most widely used pro audio laptop. Some people I know do simple mixing without plugging an external audio interface, just using built-in audio. It seems that Apple does not care about those people anymore.