Because the specification says so. Relevant parts:
15.4.4.18 Array.prototype.forEach ( callbackfn [ , thisArg ] )
...
5. If thisArg was supplied, let T be thisArg; else let T be undefined.
...
7.c.ii Call the [[Call]] internal method of callbackfn with T as the this value and argument list containing kValue, k, and O.
Now, we all know (hopefully) that if a function is to be called with a this value of null or undefined, this is set to the global object instead. Here is a refresher:
10.4.3 Entering Function Code
- If the function code is strict code, set the ThisBinding to thisArg.
- Else if thisArg is null or undefined, set the ThisBinding to the global object.
- ...
As you can also see, this will not point to the global object if we are not in strict mode (the default).
I want to understand the reason behind it
Why it was decided to do it this way can likely only be explained by someone who worked on the language. You could ask at https://esdiscuss.org or http://es-discourse.com/.
