1. Likeable characters. Yes they're cookie cutter but they're written as fun and likeable and people enjoy seeing them.
2. Big Name actors. Kind of ironic that a lot of the "big names" they ride Marvel actually turned into big names themselves but it doesn't change the fact that you can see some very well known people in these films.
3. Solid action. It's not fantastic by any stretch but marvel usually does a good job with interesting set pieces and applications of powers to provide an interesting overall visual display.
4. They're all PG-13. Sad but true.
5. Film universe. They have essentially ascended from movies to the most expensive and high-budget TV series ever. This guarantees that every film will have more audience than it would otherwise simply because people don't want to get lost in the story. I have 0 desire to Captain Marvel but will anyway so I don't miss a crucial part of IW2.
6. They're funny. No, they aren't THAT funny but usually there's at least a couple lines that even the most jaded and cynical channer will crack a smile at. However, they're HILARIOUS to the average pleb. Quipping wouldn't exist if it didn't work.
7. Cool Powers. Yes, the MCU has a lot of "Punch Man" and "Gun Man" in it but there are also the likes of Iron Man, Ant-man, Dr. Strange, etc. For comparison, literally every hero in the DCU punches people.
While it does play things incredibly safe, there is enough passion and inspiration there to elevate things beyond say Transformers. Bland and safe? Sure. But they consistently produce films that appeal to virtually everyone, you can take your entire family to and have created enough momentum and goodwill to be the centerpiece of pop-culture. And like it or not, that's how you consistently get 1B+ BOs.