Also position on a bike is different, unless you use wahoo climber or raise the wheel "manually".
Biggest thing with zwift, and it's an obvious statement, is that there is not bike handling skills that are required.
Someone mentioned descending from radio tower. Could you hit 50mph on similar road outside? Probably, it's pretty steep and there is enough straight to build up speed. Question is would someone would be able to make that left turn at that speed. My guess for majority of zwift users the answer is no.