Hell, take a look at Robert Downey Jr.’s box office pre-Iron Man (ie pretty much nothing — the closest thing he had to a hit was Back to School in 1986!). Does that guy look like he’s about to headline a multi-billion-dollar cinematic universe? Chris Evans had flopped as a super hero in Fantastic Four and done nothing much else pre-Captain America. Chris Hemsworth had a bit part in the open sequence of Star Trek but was otherwise unknown pre-Thor.
And among the directors Marvel has tapped for their massive franchises were ones who came in known as the guy who directed Elf (very good box office, but not necessarily predictive of doing a blockbuster action franchise), the guys who had directed some episodes of Community and Arrested Development, and the guy whose brother was on Gilmore Girls.
None of those people had track records to suggest they could pull off these kind of movies and Marvel was taking risks with many of them, but that’s the point. They were willing to take risks on this group (all white men, by the way) but not on any women or minorities (who would have to first be proven as big box office draws before they could get an opportunity like that).