Make up your own setting is easy for experienced players and GMs, but very difficult for beginners. And it's a lot easier for experts to ignore optional content than for beginners not having that optional content.
Maybe it is because most of us veteran DMs remember that even in our first campaign, we, too, struggled with a lot of things, and having a ready-made setting is one less thing to worry about. Dungeon-Mastering is a skill. It is something that takes work to be good at. It is something that you are constantly learning and getting better at. And it is far better to master that skill by first learning how to manage the table and ensure everyone is having a good time than to spend many hours developing a world that you do not have the skill to properly convey to your players.I'm shocked and saddened that people have such a dim view of beginning Dungeon Masters.
First, this is 4E. The world of "yes." You give me your damn backstory. All the towns you mention, all the people, all the events, they ACTUALLY OCCURRED IN THE HISTORY OF MY WORLD. Got it?
A co-create or make it up as you go is not a game that I would play in. If the DM can't be bothered to create a map, create deities, create cultures for the setting, etc., I can't be bothered to take part in the game that they are running.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.