Why was Captain Marvel not in Infinity War even though half the universe was at stake?

Because when the Avengers came out in 2012, the character was not thought of at that time and no films were planned. This really should be common sense

Fury did page her, as soon as he knew, but it was too late then (to stop Thanos).

Fury wanted to see if the Avengers could handle Loki and the Chitauri on their own, so his Avengers Initiative would get funded.

According to my kids, it's because she's so powerful that, had she been in the movie, she would have immediately fixed everything and there would be no movie. So the writers had to make up a reason for her to be elsewhere during the events of the movie.

Of course this is just their interpretation.

In ENDGAME, they try to handwave it by saying she helps out other worlds.

The actual answer is she's overpowered and would have ended the story much too early.

Same reason the Eternals weren't there. Sometimes the MCU is just making shit up as it goes.

Because she's so powerful she'd overshadow the rest of the Avengers. They wanted the film to be about the Avengers we know and love, they wanted Cap and Tony making the decisions, and not having to argue over leadership with someone infinitely more powerful.

So they saved her for the finale, really.

Because the M SHE U hadn't been set in motion yet.

It's what Captain Marvel 2 should have been. Another flashback movie showing how the entire universe was in total upheaval while Thanos was doing his bat shit crazy stuff. Properly handled, it would have greatly deepened the lore. Instead, we have whatever they're doing now. Which I think they're still reshooting this very moment?