The real secret sauce: Kevin Feige of Marvel Studios, and his team including the top-tier casting director Sarah Halley Finn.
Kevin Feige prevented Marvel movies from getting stale by completely switching the genre every single movie: eg, Winter Soldier (political thriller), Ant-Man (heist movie), Spider-Man Homecoming (high-school drama movie), Thor Ragnarok (Australia/New Zealand comedy), Shang-Chi (mythical Chinese martial arts movie).
Feige has said if a character is ever successful enough to have a third movie, they will always take a very different approach from the prior two movies. Thor Ragnarok is the best example of this: a soft reboot of the personality of a previously dull Shakespearean character into a fan favorite. The success of that soft reboot was enough to give that C-level character his fourth solo movie -- something Marvel has not done before.
None of Marvel's competitors have any producer of Kevin Feige's caliber. Not Warner Bros (DC comics), not Sony (Spiderman-villains universe) and not even Disney's LucasFilm (Star Wars).
Kevin Feige is 48 years old and it's not clear whether he'll retire after the X-Men and Fantastic Four are integrated in the MCU.
His magic touch and keen understanding of the characters have been a key part of the Marvel Studio's success, and it's not at all clear whether it will survive when he leaves.
Marvel Studios could become like LucasFilm after George Lucas sold it. It's a shell of its former self that can barely produce anything passable: Mandalorian and Rogue One just make the cut as being acceptable Star Wars, but very little else.