Control the platform, control the market around the platform.
Sure it's 'open', but at the same time, heavily subject to the authority of the owners.
See also the systemd/gnome impact to linux ecosystem..
posix in general keeps unices and unix likes (including linux here) more open than other systems since the API is less fluid / under the influence of a single group..
Sure, Google runs the core development, but everyone will benefit from this eventually.