Well there's a very obvious reason for that: we want to learn from the best. Why would you pick someone which is only good, or anything else but the best, most creative, most inspiring? Surely having an ensemble of good performers is nice, but the 'genius' by consensus is who anyone would wish to learn foremost.

I do have a specific opinion on what it means to learn form someone. People often overreach and obsess with details from the lives of those, when in reality learning from Einstein is as simple as reading what he wrote and the way he described his theories. This goes for many other geniuses -- they are geniuses in their fields, and we shouldn't expect everything they did to be right.