I guess the simple answer is that younger pitchers can be more relatable to kids than a pro. Also, not many pros have a Free detailed program online that teaches kids how to go from t-ball to HS Varsity level. Possibly because pro's are either busy or want to be paid in some way for their time.
Granted there is no revolutionary information being relayed. Most of the content is "best practices" type of stuff. But, Good information can come from anyone.
A lot of the pitching instruction videos on you-tube are narrow in scope and cover a small aspect of pitching. And you would need to piece together stuff from multiple programs to get something that's comprehensive. I'm not the kid in the video, just someone who sees the potential of the project.