How do you explain a complicated thing that you understand, to someone who could not understand it in any reasonable amount of time? You tell a story. Heck, we use metaphors all the time, and they are shortened, and less extrapolated, but still they are stories used to explain somehting that is relatively unrelated to the actual topic. This may not apply to all myths, but I think it may apply to some.
Intelligent people have always existed. Today, it may seem we have more because there are more oppurtunities to demonstrate and develop intelligence, but inquisitive, intelligent people have always been around. I find it hard to believe that more "primitive" people (say those from 10,000-50,000 years ago) didn't find the same patterns and explanations for natural events that we have discovered in the last 500 years. Those with the intelligence to discover these things without guidance were most likely in some position of power and created stories to explain these things to those who may not be able to follow his actual train of thought. Also, stories are easier for people to remember and pass on through generations.
Say a person theorized the movement of the planets and moons in a way that is similar to how we understand gravity (I doubt Newton was the only person to come to this conclusion, he was simply the first to write it down). Perhaps that person was the first in his group to actually conceptualize these patterns as a tool. Will he explain his theories about what holds everything together? Will he try to explain how he came to his conclusions? Probably not. He would be far more likely to say that the gods have told him a story. He would tell a simple story of how the stars relate to each other at different times of the year, and how to use this information. The story would become more detailed as other people picked it up, and eventually a rich series of stories would become part of an oral tradition.
This may not apply to most stories, but that is how I always imagined some myths to have come about. We are all inquisitive and we always have been. We have always felt the need to accumulate information and give it to the next generation. If you aren't writing things down, stories are the best, most effective way of ensuring posterity.