Contrast, for a moment:

    var x = fib(a, b);

with

    (let x (fib a b))

And count the delimiting symbols. I count 5 in the first example. In the second, I see 4.

Let's try something a bit harder.

    function(a, b) {
        return a * b;
    }(6, 4);

vs.

    ((fn (a b) 
        (* a b)
    ) 6 4)

10 first, 8 second.

Yes, "why all the [symbols]" indeed.