Why did the authors of this myth not give Merlin perfect powers of prophecy?
First, The Once and Future King by T. H. White, is a modern novel, taking as its subject matter the corpus of Arthurian legends. It itself is not a myth, or even a transcription of oral epos, but rather a twentieth century work of historical fiction.
Within the legends on which White based his tale, prophets generally were not expected to have complete knowledge of the future or to communicate it unambiguously. Just like the Old Testament prophecies, the ones in the Arthurian cycle are ambiguous.
One reason for this is that complete and accurate prophecy might run into problems with free will. Also, as a matter of literary technique, limitations on detail in prophecy allow the author to generate more suspense, even when the general outlines of a plot will be familiar to literate readers.