Because people tell you their name if they want to, and give you their thoughts if they choose to do so. Having your name available to all at any time and anything you've ever said regardless of context or without limits to the intended audience is madness. It's a freedom we should not be willing to give up so easily because of technology.
On top of that, having all that personal information available will actually increase the power of trolls and political manipulation, not to mention exacerbate identity theft, profiling, security and phishing problems.
You don't have to give your name to anyone. Single sign in implies a single person but doesn't need to have any more information on that person and even if it has the info doesn't have to share it with anyone.
On top of that, having all that personal information available will actually increase the power of trolls and political manipulation, not to mention exacerbate identity theft, profiling, security and phishing problems.