Why is `this` a pointer and not an rvalue reference in C++?

First, your question should really be: "Why is this a pointer instead of a reference?" (Note: that's reference and not rvalue reference.)

The answer is simply because Bjarne invented this before he invented references.

Member functions appeared in C++ long before rvalue references (C++11) and even just references. But in C++23 with Deducing this proposal, you can do the following:

struct Foo {
  void bar(this Foo&& self) {
    // self is rvalue reference to Foo
  }
};
// ...
Foo foo;
std::move(foo).bar();