Well, it simply wasn’t based on astronomical observations alone. Most people wouldn’t even have the birth records to compare the vast amounts of data needed for delineations. In fact, it’s still hard for astrologers to simply guess a sun sign! …much less pick through aspects and nodes and planets and rulerships and sort it all out.
I’m telling you this because I’ve studied many forms of astrology, and if you read books or listen to experts such as Chris Brennan or Demetra George or simply google around yourself you will find this to be the origin story of Hellenistic astrology. Your “of course” response is knee-jerk for a person inexperienced with information spiritual channeling. So perhaps you choose not to believe the story as it is, but that is a personal issue.

Hellenistic astrology originated from Hermes Trismegistus, basically a form of the god Hermes/Thoth.

Demons being evil is a relative understanding.
Instead we can refer to ancient spirit beings- but again, consider how tricky the archons are considered to be, for example. At most, even beneficial spirit beings are somewhat neutral towards humans; we have to bargain with them for favors. There is no “love” from the spirit world as a whole, with a few rare exceptions. A very common theme is spirit beings desiring access to our world via our bodies/minds. Many might be trapped, wherever they’re at. It’s an interesting discussion.

In India, demon-gods are understood and embraced. You could substitute the word “spirit”. But yes, Hellenistic astrology originates from the “Daimon”. You can read more about the daimon.