I didn’t say he originated it. I’m saying he became the gold standard for talk shows, and everyone making their own show used the Tonight Show as a model. Johnny may have copied it from others (just like he copied many of his comedy bits), but they all became associated with him.
Only in the minds of youngsters. 
I always assumed it’s like lots of things on TV that occur left to right. It’s because western viewers read left to right, so action that takes place left to right is more natural to the eye.