Cavill bailed because he's a fan of the novels and games and didn't like where the show was heading.
Funny thing was it was around the same time as his Superman cameo in Black Adam, so the internet's best and brightest thought he had left a show to return to Superman even though there was no contract and no movie (and even if there was a movie, it would be years away).
I never thought he left just to do Superman. That would make no sense, like you pointed out.
It WAS really shitty how WB pulled the rug out from under him, though. Makes it almost impossible for me to want to support James Gunn's Superman film