Because they are not isomorphic. At all. Even if you just consider the languages themselves, and ignore their ecosystems, which, in practice, you cannot.
For machine manipulation, I think it makes more sense to directly manipulate the AST.
For human manipulation, I think the cognitive overhead of mentally converting between the display syntax and the canonical syntax would far outweighs any gains in readability. But maybe your workflow is different than mine - If you have a lot of custom macros in your editor, I could see s-exps being useful (although, again, I think exposing and directly manipulating the AST would be less error prone)
If you designed the languages and interpreters/compilers around it. Neither Python nor Javascript are, though.