How can I count all words or characters in a Powerpoint pptx file? Is there a simple way to count the characters of all text fields in a Powerpoint presentation?

I tried exporting to Rich Text Format (.rtf) to do the count in MS Word, but the export function only exports some text fields (outline text) and misses small annotations, lists, etc.

Some tools like Count Anything or Anycount are not available for Mac OSX.
No, the way is not at all simple. Specifically, the file format is complicated so analyzing the file won’t get a quick answer.
To answer your question, it's helpful to know that the .pptx format is actually a zip file full of .xml files. I found folders called slide1.xml, slide2.xml, etc... but there are many others and the text within them is surrounded by the usual xml formatting tags.
You can unzip by replacing .pptx in the filename with .zip (you may have to enable visible extensions) and unzipping with something like Keka. I've had trouble unzipping with the native Mac OSX Archive utility, but Keka has always worked for me.
For more detail, see this page: Your slide deck is a zip file in disguise