It was a mediocre movie anyway

Maybe I'm looking at this the wrong way, but why do people say or write that something makes them "speechless"?

I.e., "I'm speechless!"

No, you aren't speechless. You managed to tell me that you're speechless, which means that you aren't, in fact, speechless.

Unless they mean that they don't have a prepared speech on the matter.

But that should be obvious because they're not carrying around a Teleprompter.

I just don't get it.