Because Google is heavily employee-driven and trusts individual employees to determine what's relevant to solving the problems at hand.
My teammates calendar topics are visible to me so that I can notice if they're getting pulled into meetings on topics that I should be meeting with them also. My boss's calendar topics are visible to me so that I'm empowered to ballpark the relevance of my project in context of other things my boss is managing (and so I can notice if my boss is working on something that should be relevant to my project that nobody has noticed is relevant yet).
My teammates calendar topics are visible to me so that I can notice if they're getting pulled into meetings on topics that I should be meeting with them also. My boss's calendar topics are visible to me so that I'm empowered to ballpark the relevance of my project in context of other things my boss is managing (and so I can notice if my boss is working on something that should be relevant to my project that nobody has noticed is relevant yet).