From asking myself a lot of the questions you did, here are my conclusions:
1. People (as in our entire species) are drawn to try and find meaning in life. We have the intellectual machinery to conceive of the passage of time and the death of everyone we love including ourselves. We can typically get these ideas by age 3-4. It is an amazing ability, but there are side effects. Like grappling with reality itself. Religion and spirituality have popped up and continue to pop up again and again, in societies that had no knowledge of each other. It's kind of a universal human feature to grapple with our earthly existence in some way. Some of us resort to becoming hyperlogical (in a futile and irrational attempt to control the world and predict the future), some seek out spiritual wisdom that actually addresses the big questions of life, and some find a blend. Becoming hyperlogical to the point of dismissing the very real purpose of spirituality and religion entirely, is just as silly as believing that God exists. Both deny something fundamental. But the behaviors serve the same purpose: they make it easier to face down the big questions of life (or to try to ignore them).
2. If people's early experiences are so important to who they are as adults, why wouldn't it be feasible that the varied nature of the seasons have some effect? And, if people have been practicing astrology and drawing insights based on many people who were all born around the same time, might they actually get something right amidst all the noise? My point is, there may be something to the idea that someone born in the middle of summer would develop differently than if they were born in the dead of winter. Maybe some of the associations are true.
With all that said, I still bristle at people who seem to take astrology too seriously. Same as I understand why Christianity exists and is so popular, but have a hard time dealing with the most devoted individuals. Still, there is surely wisdom there. A lot of therapists even advocate for their clients to pull tarot cards, not to get wisdom from the cards, but from their own intuition.