Call me a cynic, but I honestly do not believe it is the ease or difficulty of DMing that makes someone choose to do it. People DM mainly for other reasons-- they like creating stories and outlines and plots, their personality is such that being the one "in charge" comes naturally to them, they want to play bad enough that they will run a game just so there is a game to be played, they are a multitasker at heart and enjoy the challenge of juggling many balls at once (or as @overgeeked mentioned they have ADHD and are just wired to perform better doing many tasks rather than just one), or they have played enough that they've developed a very specific or idiosyncratic preference towards gaming and running games is the only way to assure themselves they can get the type of game they want.
It doesn't matter how easy you make it... most people just don't want the responsibility of running the game. In fact, I would go so far as to say that the type of person who would want to DM would strive to do so regardless of how easy or hard it was. If the act of DMing was that enticing to someone, they're the kind of person who will do all the required legwork and reading necessary to learn how to do it. After all... who are the people who usually are the ones we hear about who don't learn the rules? The players. The DMs almost always do learn the rules, because acquiring that knowledge is part and parcel with wanting to acquire the ability to DM. Few are the DMs who don't learn how the game works and are still able to maintain a table of players (at least a table of players who do care how the game works.)
So while making the rules and instruction clear on how running the game works and starting simple and moving towards more complexity is never a bad thing... I also don't think we or WotC needs to go over the top with it. Because quite frankly I am adamant in the belief that the more you try to parcel out the rules just like they were board game instructions, the further you get away from what roleplaying games are. Some of you might not like "Rulings, Not Rules"... but Rulings are exactly the thing that makes a roleplaying game DIFFERENT that a board game. Because you CAN'T have a rule for everything in an RPG because no game can guess the infinite number of things that the players will think to do. The game can give suggestions on how to adjudicate actions, and indeed that's part of the necessary instruction the rules should pass on to people... but thankfully the type of person who would be predisposed to dungeonmaster is probably the type of person who would be okay with doing so.
It doesn't matter how easy you make it... most people just don't want the responsibility of running the game. In fact, I would go so far as to say that the type of person who would want to DM would strive to do so regardless of how easy or hard it was. If the act of DMing was that enticing to someone, they're the kind of person who will do all the required legwork and reading necessary to learn how to do it. After all... who are the people who usually are the ones we hear about who don't learn the rules? The players. The DMs almost always do learn the rules, because acquiring that knowledge is part and parcel with wanting to acquire the ability to DM. Few are the DMs who don't learn how the game works and are still able to maintain a table of players (at least a table of players who do care how the game works.)
So while making the rules and instruction clear on how running the game works and starting simple and moving towards more complexity is never a bad thing... I also don't think we or WotC needs to go over the top with it. Because quite frankly I am adamant in the belief that the more you try to parcel out the rules just like they were board game instructions, the further you get away from what roleplaying games are. Some of you might not like "Rulings, Not Rules"... but Rulings are exactly the thing that makes a roleplaying game DIFFERENT that a board game. Because you CAN'T have a rule for everything in an RPG because no game can guess the infinite number of things that the players will think to do. The game can give suggestions on how to adjudicate actions, and indeed that's part of the necessary instruction the rules should pass on to people... but thankfully the type of person who would be predisposed to dungeonmaster is probably the type of person who would be okay with doing so.