Microsoft is opening their codebase because they've been crushed by the enormous efforts of open source. This concession was hard won over the course of decades.
Apple and Google do not contribute code freely, nor do they contribute a significant amount of their code. The contributions are limited to areas in which an advantage exists. One only has to look at the machinations present in other development platforms to realize the threat. Consider what's happened with Java in recent years. Or look to Swift. Or to the entire Microsoft ecosystem which was built in part on a foundation of open source. BSD licensed code permeates the Windows environment; to ask "what's the concern" belies a rather stunning ignorance of Microsoft's behavior over the three prior decades.
Apple and Google do not contribute code freely, nor do they contribute a significant amount of their code. The contributions are limited to areas in which an advantage exists. One only has to look at the machinations present in other development platforms to realize the threat. Consider what's happened with Java in recent years. Or look to Swift. Or to the entire Microsoft ecosystem which was built in part on a foundation of open source. BSD licensed code permeates the Windows environment; to ask "what's the concern" belies a rather stunning ignorance of Microsoft's behavior over the three prior decades.