Python has a different "philosophy" than "C++ et al". Literally everything is an object, and everything has attributes; some of those attributes are objects that implement the Callable protocol. And it's a syntactic sugar that some of those callables are implicitly passed certain predefined arguments; the default is passing the instance of the object itself. The exact same thing could have been implemented without it, like this:
foo.some_callable_attribute(foo, another_argument)
(1) + 2
(1).__add__(2)
int.__add__(1, 2)
I think a "magic" this keyword can create a lot of nasty edge cases that can be difficult to reason about; the way "this" is used in JavaScript is notoriously complex in ways that it might not be in a statically typed language like C++. What should "this" evaluate to outside of an instance method? What about in a class definition inside an instance method? What if an instance method is called directly instead of on an instance? All of these situations require making their own "rules", whereas in Python the correct answers can be easily reasoned about by starting from first principles.