I generally don't know what my installers do. When I install games from steam, applications from the App Store or browser from Mozilla, or Python distributions

I generally don't know what my installers do. When I install games from steam, applications from the App Store or browser from Mozilla, or Python distributions from continuum I don't know what the installer does. Or really, the software for that matter. How would I? And even if I did, how would I know what the software does? If you don't trust GitHub user robbyrussell to serve you a legitimate installer, why would you trust that user to serve you a legitimate program? The part about partially downloaded scripts causing havoc is however quite relevant.