The Marvel Cinematic Universe (MCU) is known for its interconnectedness, with characters often appearing in unexpected movies or making surprise returns.
This interconnectivity has been a hallmark of the franchise, as seen in the unannounced appearance of Robert Downey Jr. in The Incredible Hulk and the recurring appearances of Samuel L. Jackson.
The studio made this clear when Robert Downey Jr. appeared unannounced in The Incredible Hulk
Author's summary: MCU's interconnectivity may ruin Ultron's return.