> This code does exactly the same thing as Rust code

Except it doesn't - because after the final parenthesis of the Lisp snippet, the let-binding is finished, i.e., the introduced variables are no longer in scope. That is not the case after the end of the last line in the Rust example.

The whole point of parentheses is to mark where something starts and where it ends. Here, it is where the scope of the let-defined variables starts and ends. Without some sort of "parenthesizing", there might be a start, but it's much harder to mark where something ends.