It was proven to be successful yet they had a colossal brain fart I guess.
Iron Man, The Incredible Hulk, Iron Man 2, Captain America, and Thor all came out before the first Avengers movie. None of which had characters interacting with each other. The first time they meet, is in Avengers. (Besides Tony and Natasha) They had built a foundation and ground to build on.
DC should have done something similar to that but it felt rushed and uninspired. They should have done Man of Steel, a Man of Steel sequel, A solo Batman film, maybe a sequel, Wonder Woman, aquaman(before JL), and maybe a Green Lantern film. Thennn they could have brought them together for JL. All they had to be was cohesive. The BVS film should have never been made.
Batman and Superman are more popular than probably 95% of the Marvel universe and they still dropped the ball. It could have been as epic as the MCU if not more, but they didn’t let the audience connect with the characters in the way the MCU did.