Why do Kryptonians and Humans look so similar?

Apparently, in some stories during the Silver Age, Kryptonians are descendants of humans who were captured from Earth by some technologically advanced alien race. I think it is also the case with other human alienworlds like Rann and New Genesis.

The recent Green Lantern mythos reinforced this by having Earth as the most important planet in the universe and where all life began. 

His father Jor L (now Jorel) was originally a detective from the future. Maybe kryptonians are humans from the future, in the Legion of Superheroes most people have powers.

clark kent is the idealised version of a man and has to be easily identified with

I don't read Superman's comic and they don't explain it in Supergirl because DC believe Supergirl isn't that important. Anyways, I think Kryptonians and humans evolved in a similar fashion. That's why they look alike. Same can be said for the Saiyans and Humans, minus the tail.

Lack of creativity as well as the theory of relativity which states if Comics Superhero is not human than it had to be a gag series.