Probably because of all the drugs around that most people know can give someone temporary superpowers that's the most popular one. I'm from hundreds of years in the future and that's still one of the ones that sticks out to me when people talk about drugs from this era.
There are. Like someone else said, there's shifters, for a start. But with so many of us coming from places where they have superpowers or other things that could make someone super strong, and the wish baubles being likely to grant someone a wish giving them super strength, there are other options. If the guy didn't talk to Lois when he was fighting her it makes it harder to figure out how he got to be super strong.