Then again, this is the Avengers we’re talking about, so logic doesn’t necessarily have to be on the table.
The public may think that he’s stuck in another dimension, or has firmly committed himself to spreading patriotic goodwill for the United States across the cosmos.
That’s the most logical answer, but it’s interesting that nobody is saying it if that’s the situation.
It’s probably fair to assume that the world doesn’t know the truth about Steve Rogers’ return to the 20th century (he probably would have been at the Smithsonian unveiling if they did), so is it possible that they think he died a hero’s death along with Tony Stark?