Earlier quoted context omitted.
> If you read a few books on mathematics you think you're easily going to become one of the top mathematicians? No, but so what? The guy behind 3Blue1Brown probably isn't one of the top mathematicians of his era. But he's having quite an impact. He turned explaining fairly basic math concepts in mathematics into a lucrative job. And who wrote the textbooks you're referring to? Probably not any of the top 10 living ma…
All those people you mention have studied mathematics for a long time, Grant Sanderson has probably studied it the "least" but that still means a BSc in his case which is not something you accomplish in a weekend. All the textbook authors are usually professors.
Textbooks used in college coursework are usually written by academics, for obvious reasons. Plenty of independent learning / pop textbooks are written by "normal people" who aren't tenured professors.