why don't const and let statements get defined on the window object

1. Why do variables declared using the var keyword get defined on window and variables declared with const and let not defined on window?

Because the specification says so. If you are asking for the reason behind that decision, then you should reach out to the specification maintainers.

Classes do not become properties of the global object either btw.

2. What is the difference between "global scope" and the window object provided to us by browsers.

There are two types of base environment records according to the spec:

• Declarative Environment Record
• Object Environment Record

A Declarative Environment Record is basically your standard environment that you get when calling a function. All bindings (variables, constants, etc) are defined in some internal data structure that cannot be accessed from normal code.

An Object Environment Record on the other hand uses an actual JavaScript object to store bindings. This is used for example by the now deprecated with statement:

with({foo: 42}) {
  console.log(foo);
}

Now, a Global Environment Record actually consists of two environment records: A declarative environment record and an object environment record. The object environment is backed by the global object, i.e. window and contains var declarations and other globals that the browser provides. The declarative environment contains the let, const, class, etc declarations.

From https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const

This declaration creates a constant whose scope can be either global or local to the block in which it is declared. Global constants do not become properties of the window object, unlike var variables. An initializer for a constant is required; that is, you must specify its value in the same statement in which it's declared (which makes sense, given that it can't be changed later).

To answer your 2nd question, global variable can be accessed through window object. However, there is an exception for const.