Sounds easy, but the reason they didn't do that is because it's extremely hard to make a good movie that most people enjoy and makes a profit, just so that you can then take those characters and put them in an even bigger team up film. What Marvel pulled off is a miracle. Universal tried that idea twice with the Dracula film and the Tom Cruise Mummy film, and both failed, killing that cinematic universe, after they had announced a new Frankenstein film with Javier Bardem and the Invisible Man with Johnny Depp.
They even took a group pic....
that will always be the saddest promo pic ive ever seen...