The ancient stories of the Upanishads and Puranas derive their charm largely from the fact that they use exceedingly simple, sometimes even childlike images and metaphors to explain the most profound truths of human existence. One could argue that they must have functioned in their time somewhat like the formulas in modern physics: hyper compact statements of truths that need extensive explanation before they can be fully understood.
There may have been several reasons to keep such stories short. Even long after writing was invented, manuscripts were time-consuming to make and difficult to keep. So, to be preserved, texts were memorised and recited, passed on with a lot of care and respect from one generation to the next. Possibly related to this, the word was still considered sacred, to be used sparingly, and known to have power. For the most sacred texts, oral transmission was preferred over the written word.
As a result of all this, every little detail, however trivial it may look at first sight, has been included for a purpose, and can point, for those who can decipher it, to a deeper reality behind.