I actually remember that in an old interview (that I can't find sorry) Grant Sanderson said he moved in to YouTube because he thought it was a more promising career than being mathematician, it's interesting to see his opinion change.
Hmm, that seems like a pretty cynical point to make. He still deeply is in the field of mathematics, as a math educator. He even makes this exact point in this video, that communicating math is still a hard and respectable part of the profession.
I remember a post in... Reddit I think it was? where I commented on an early video discussion that he should teach online full time, because that kind of intuitive education was sorely needed. He actually answered the comment, along the lines of "that would be cool but I already have my hands full in $UNIVERSITY".
I think it was relatively soon after that interaction that he started doing youtube full time. I've always wanted to think I made a teeny tiny contribution to move the needle there :)