Why do all these meta humans wear their briefs on the outside?
Superman artist Joe Shuster was inspired by the circus strongmen of his era.

Later characters were inspired by Superman.
One in particular is supposed to have inspired him, Zishe Breitbart. In the children’s book about him I used to read my granddaughters, there are pictures of him in a red cape, but none in the trunks over tights thing. In the image library available on DuckDuckGo, there are pictures of him in trunks, but no tights. Must have been other strongmen in the mix.