Since most of these companies do not sell their software per se (Microsoft being the prominent exception) and maintain their position in the market through other means (network effects mainly), they see no harm in open sourcing their work.
But if the bulk of software innovations are coming from such companies, what does it imply about the state of the industry then?
It has already led to a peculiar situation where programmers perceive the best jobs to be in companies that do not sell software at all!