For an important event, such as at your Bar Mitzvah or Bas Mitzvah, it’s up to you to speak to the guests. This speech is not something you’ll think of on the spot. You will invest a lot of thought and research into it, and you will probably get some help preparing the speech.
Speaking of speeches, there are some people, such as kings, presidents, and prime ministers who employ full-time speechwriters for themselves. These professional speechwriters are creative and talented and know just how to fill the speech with the right ideas, stories, and even jokes.
But there was no speechwriter for Moshe Rabbeinu (and no jokes). These speeches were straight from his heart. The previous 4 books in the Torah are full of HaShem’s words, which Moshe faithfully documented. Devarim, our Rabbis of the Talmud (Megillah, 31a) say, “Moshe said it on his own.”