כי כבד פה וכבד לשון אנכי For I am heavy of mouth and heavy of speech
The Pasuk tells us that Moshe was trying to find reasons to demonstrate that he wasn’t qualified to be Hashem’s spokesman, citing that he is heavy of mouth and of speech. The Ramban notes that this issue was easily resolved as Hashem could have cured Moshe from his heavy speech. However, since Moshe, due to his humility, didn’t want the position, he, therefore, never davened to be cured. The Ramban explains that instead of removing Moshe’s speech impediment, Hashem told Moshe Rabbeinu to use words that he could easily pronounce.
The Sam Derech notes that the reason Moshe did not pray to remove his blemish was because he didn’t want to become the leader of the Jewish people due to his humility. If so, then precisely because of his humility, this should have been the very reason that he would merit to have his speech repaired. The Sam Derech explains that Hashem created the world in such a way that He wants us to recognize His control over the world. Unless a person feels the need for something and makes a request (davens) for it, Hashem will not necessarily provide that item. Since Moshe Rabbeinu never davened to have his speech impediment removed, Hashem didn’t remove it.
Rav Shimshon Pinkus in his introduction to his work Shearim Bitfilah develops this idea in great depth by explaining that even things that we have merited still cannot come to us unless we daven to Hashem and express our desire to have them. He demonstrates that we find this idea earlier in the Torah. Hashem promised Avraham Avinu that he would have a child through Sarah, and that child would have children, such that Avraham would have descendants as numerous as the stars in the heavens. Even though his father had been promised that he was going to have children, Yitzchak, the only child of Avraham and Sarah, could not have children until he davened to Hashem to request them.
We see from this that davening shouldn’t be considered only the mechanism to ask Hashem for things that we don’t deserve. Even if we are supposed to have something, without prayer Hashem does not send it to this world. We should remember the importance of prayer, and of always calling out to Hashem to request everything.