I've actually given this some thought as to just why it happens, and I've come to suspect that...
a) ...because the church leadership becomes convinced that every little imaginative flash of "insight" that pops into their collective heads is divinely-ordained (and no one tells them otherwise, 'cause they've surrounded themselves solely with people who are never allowed to disagree with them), they simply assume that any evidence that become apparent will naturally vindicate them, and...
b) ...back when the theology was originally crafted, the world was so much bigger and science was so much less advanced that disproving a belief with actual hard evidence (as opposed to verbal testimony) was way more difficult... and therefore - for all intents and purposes - not worth worrying about.
I could be wrong.