Why do promoters of astrology bail when asked for data sources?

Your weird obsession with the supernatural is so funny

I'm not sure what you mean by data sources.
Do you mean original medieval texts, from Europe, India and China, from which the modern practices were derived? I'm not even sure whether those still exist.
If not, do you mean the data for the changing relative positions of planets and constellations? Astrologers draw that directly from astronomy. They even change and adapt their system when a new planet like Uranus is named.

I agree that the belief system itself is cockeyed foolery. But people love to believe things - and often they choose a specific belief because it fits their chosen identity as "rebel", "eccentric", "hippie", "new ager", "alternative" or whatever.

I think it works as a party game. One can use it to create a jokey conversation, without getting too personal with a stranger, and it reveals something about what they do and don't believe. So it already reveals something quite significant.

If you don't believe it yourself, why don't you just shut up about it and stop annoying people? Why go on and on and on about something you don't believe in? It only makes you look like a troll.