How is because people over history ensured it was here for people today. Why is the question you should be asking. And also, what, what about this mythology is relevant to me or me studying astrology?
If you spiritually advance to a point of sensitivity and meditate on these questions you will receive the answers in the mythology, but you can see the patterns that astrology dictates in everyday human life if you become aware enough of them. Then your everyday routine will observe the "mythology" of your town or wherever you live, lol.
The Gods left us these stories so that when things were tough, we had some cultural forms through which we could still psychically aid ourselves with in the expansion of our knowledge. Im sure you know they left allegories relating to things like the chakras in a lot of their mythology. Like with anything fanciful some people took this for literal especially in the post Roman times, and this came with the denunciation of the spiritual. Now that we have modern culture and the internet we literally have the JoS, so the mythology for a lot of people is thrown out the window because they get overexcited at the direct knowledge it gives. Astrology is one thing which benefits from it though but like with the poppits and ouija boards the stories can be put in a similar understanding that you don't need the stories to understand astrology it just makes it easier sometimes. You dont need poppits to do magic but--. It's a similar kind of thing to that, mythology is not 100% important just the expression and its allegories which you don't exactly need if you are fully adhering to the eight fold path of Astarte for example.
The stories contain allegories which help you understand how the planetary or stellar body relates or affects the human form in different ways. When astrology becomes a science we might lose these cool little stories, but for now we've still got a while lol.