We saw Thanos three times before Infinity War, two of which were post credit scenes where he had 1-2 lines, and the other was Guardians Of The Galaxy. When it comes to the concept at the center of the movie, we saw the Infinity Stones (including retconned stones but not appearances of characters with the stones/fake ones like Vision) eight times in these projects (before Infinity War):

1. Thor
2. Captain America: The First Avenger
3. The Avengers
4. Thor: The Dark World
5. Guardians Of The Galaxy
6. Avengers: Age Of Ultron
7. Doctor Strange
8. Thor: Ragnarok

As for the Multiverse, we've seen it in (Not counting non-canon projects like YFNSM and Marvel Zombies):

1. Loki Seasons 1-2
2. What If...? Seasons 1-3
3. Spider-Man: No Way Home
4. Doctor Strange In The Multiverse Of Madness
5. Ant-Man And The Wasp: Quantumania
6. The Marvels
7. Deadpool & Wolverine
8. The Fantastic Four: First Steps

That's the same amount of projects, and while we've only seen Doom once, in First Steps, it's nearly the same amount of set up for the movie.
People are talking about how much setup Infinity War had in comparison to how much Doomsday is getting, but they fail to realize that we barely saw Thanos before that.