The two characters had a major falling out in Captain America: Civil War, and three years later, they are still not on speaking terms.
Perhaps the biggest unresolved MCU plot-line right now — other than half of all life in the universe getting wiped out — is the continued beef between Captain America and Iron Man.
Plus, nobody wants to see these guys go out on bad terms!
The ball is in Tony’s court to make the first move, but even with the fate of the universe on the line, he still hesitated to make that call for help to Cap.