Fans might be delighted that Henry Cavill is returning as Superman in future DC movies, but Warner Bros., the studio who makes the films, reportedly has no faith in Cavill's ability to headline a solo Superman film. Going all the way back to 2013's Man of Steel, theactor has starred in a series of films that have been rumored to have underperformed relative to Warner Bros.' expectations. Man of Steel earned $668 million globally, making it nominally the highest-grossing Superman movie -- although adjusted for inflation the original Superman: The Movie's numbers are astronomical. The studio followed that with Batman v Superman: Dawn of Justice.
When critics claimed that Warner wasn't happy with the numbers on Man of Steel, the studio denied it, pointing to the immediate greenlighting of Batman v Superman, which retained Man of Steel director Zack Snyder. After Batman v Superman got scathing reviews and didn't crack $1 billion (something that the previous two Batman movies had done before that), Warner admitted they were disappointed, but powered forward with plans for Snyder to direct Justice League.
That film also featured Superman, albeit in a much more limited capacity since he gave his life stopping Doomsday in Batman v Superman. And between studio meddling, the departure of Snyder due to a family tragedy, and a tone that was charitably described as all over the place, Justice League was such a dud that it threw the DC films into crisis for a while, with only the successes of Shazam! and Aquaman convincing the studio that there was something worth saving there.
Cavill has not appeared onscreen since 2017's Justice League, although Superman had a brief cameo (from the neck down) in Shazam!. In the time since, rumors had begun to circulate that Warner might try to make a feature film based on Supergirl instead, to keep the Kryptonian legacy alive on the big screen without relying on Superman. That plan went to the back burner when Cavill returned for more DC roles, but the studio reportedly wants him in a role comparable to Mark Ruffalo's Incredible Hulk -- used to make other people's movies better, not to headline his own.
That's according to an anonymous source cited by Heroic Hollywood, who claimed, "A stand-alone Superman movie would not be successful at this time. Perhaps once there's more momentum after playing a supporting role in some successful DC films, there would be more of a chance for a stand-alone film." Anonymous sources are never ideal, but this tracks with much of what is already known about Warner's current DC plan, so it is likely true.