I think that knowing that you can craft a ruling in the moment instead of knowing all the intricacies of rules is key to success of a GM. And in 5E D&D, in particular, this is something that is brought up often. It's one of the big things that the DM should try and remember above all other things. Knowing this approach and relying on static elements like Character stats and DCs will get you pretty far as a DM.I think GMing is different from playing, in ways that tend to make it more difficult (or at least more complex, which isn't exactly the same thing). The GM is, in many games, the final authority on the rules for that table, which implies an expectation to at least know the indices, if not the entire books. While the players are usually responsible for one character each (sometimes players run multiple characters), the GM is responsible for the world. Even in a published adventure, the GM needs to keep straight what is going on offstage, and know what a given NPC's motivations are, and where things are in the neighborhood and in the world. Some people will find the complexity more daunting than others, some will find it more difficult than others.
I agree with you that the GM typically has more to do than a player. This is why I advocate offloading some of the non-DM-essential tasks to others.
I understand the sentiment, but I think that framing it this way creates a dichotomy that's unnecessary. The role of player and GM are equally important, as the game cannot occur without either (except in games that have been crafted with that in mind, like Fiasco or similar games). The fact that there's almost always more than one player and almost always only one GM is what shifts that balance a bit.The players do not typically have the same level of responsibility for the game as the GM does. However, I would certainly agree the players do have some responsibility and they're certainly important.
Related, having a game with the best GM ever might still fall flat if the players are simply going through the motions and not bringing any creativity or energy to the game. Flipping that, all the energy and creativity in the world on the part of the players can only do so much if the GM is simply going through the motions.