twofalls
DM Beadle
I certainly understand your point, and it's a good one.Not sure this explains why watching other D&D groups is popular, but it may directly answer the question of why, as a DM, you SHOULD watch.
Great Authors voraciously read other authors
Great Filmmakers watch hundreds or thousands of films
Great Musicians listen to a variety of styles and composers
Great Painters study other's art
Great Video Game designers play other games
Before streaming and podcasts, the ability to expose yourself, as a DM, to a wide variety of other DMs work was difficult, if not impossible. With streaming, you can easily do it. And the games you watch whose playstyle you don't like personally can be just as important to improving your game as the ones whose style you do like. To a lesser extent, it can also help players become better as well, by watching how other's play the game and incorporating ideas that appeal to you that you'd never thought of.
I've run games for four decades and I will never be perfect, but the idea of spending time doing something I really dislike in the hopes of improving my performance in something I do because I enjoy it seems rather counterintuitive.
After 40 years I have learned that it's very hard to make myself change my play style no matter how much I might improve by doing so. My players keep coming back, so I must be doing okay.