Dave sells video views, simple as that. If he steps them up to the level of PhDs, the audience shrinks to the point that there is no money in making videos. The videos need to be of value to beginners.
I would not get those things from making advanced level theoretical videos. If you want that, MIT have their entire course available on their Youtube channel for free, I recommend you go watch them instead of my channel.
In my personal opinion, Youtubers are for ENTERTAINMENT only, think about ElectroBOOM, maybe the biggest in this topic.
The Signal Path, EEVblog, and others are good if you Already know electronics, but they are useless for teaching electronics, most of the time, are just curious entertainment of something that you will never use in your life.
Very few YouTubers are good to teach electronics: w2aew (advanced), GreatScott! (intermediate), learnelectronics / electronzapdotcom (beginner).
But, for me, the most effective way to learn electronics is through Books, maybe university videos; other resources, like this forum, are for very specific or practical issues.
edit: or for hacking something