Why does (function() { return this; }).call('string literal') return [String: 'string literal'] instead of 'string literal'?

.call internally invokes [[Call]], which performs

6. Perform OrdinaryCallBindThis(F, calleeContext, thisArgument).

And OrdinaryCallBindThis (which sets the this value of the function that's going to be called) will wrap the new this in an object in non-strict mode:

5. If thisMode is strict, let thisValue be thisArgument.

6. Else,

   a. If thisArgument is undefined or null, then ...

   b. Else, Let thisValue be ! ToObject(thisArgument).

In strict mode, you'll see that the string does not get wrapped in an object:

'use strict';
(function() {
  console.log(this);
  console.log(this === 'string literal');
}).call('string literal');

Since you are calling a method on the string primitive, it is automatically converted to a String Object.

The logs are how the two consoles render String Objects.

Further reading: What is the difference between string primitives and String objects in JavaScript?