There are numerous fixed points on our radar maps (VORs, NDBs, reporting points, centrelines etc) that aircraft fly over and that would soon show up any inherent innacuracies if pilots were reporting over them and the radar was displaying them a significant distance away from them. It doesn't.
Some primary radars do use known fixed ground targets for calibration/set-up checks.
In the case of the two mile wide return on an unprocessed primary, you work using the centre of the return as being the position of the aircraft, not the whole two mile wide blip.