Why can't you initialize the key for an object literal, by calling an array element? (Javascript)

Question

let arr = ["a", "b"];  // (2) ["a", "b"] 
let obj = {"a": "b"};  // {a: "b"}
let obj2 = {"a": arr[1]};  // {a: "b"}

let obj3 = {arr[0]: "b"};
    //VM625:1 Uncaught SyntaxError: Unexpected token '['

When initializing an object literal, why can't I populate a key by referencing an array element?

(Javascript 1.7)