It's not hearsay to go to the witness stand and say the defendant told you something. That defendant is there in court and is able to defend themselves.
Laws are generally clearer if you write them as general definitions and general rules and if those cover too much carve out exceptions in the rules rather than the definitions. It would work to make the exceptions to the definitions instead, or to both the definitions and rules, but that is usually going to be more complicated and less clear.
For hearsay the general definition is "an out of court statement offered to prove the truth of what the statement asserts" and the general rule is "hearsay is not admissible", and that is narrowed by a bunch of exceptions to the "hearsay is not admissible" rule. As far as I remember pretty much everyone has left the definition untouched.
Similar motivation as exceptions for error handling
> The defendant in a criminal case is always available, so any statement they make out of court is never hearsay.
That's also not quite correct. A declarant is considered to be unavailable for example if they refuse to testify about the subject matter despite the court ordering them to. They are also considered to be unavailable if they testify that they cannot remember the subject matter. See rule 804(a) of the Federal Rules of Evidence for the criteria for being unavailable.
(I'm using FRE because that seems to be what others are using. Really though we should be using Missouri's rules of evidence. I think most state's rules of evidence are fairly close to the FRE so that's probably reasonable).
> if Susan is unavailable for cross-examination, the answer is hearsay