Why are there UFOs in many Renaissance paintings?

Carlo Crivelli, The Annunciation with Saint Emidius (1486)

As Jacques Vallee revealed in the classic Passport to Magonia, the UFO phenomena can be traced all the way back through recorded history—we just called them different things. For example, check out Artnet's fascinating collection of Renaissance paintings that depict mostly biblical scenes featuring Unidentified Aerial Phenomena (UAPs). Above, as described by Artnet:

Crivelli's luxuriously detailed work offers his artistic interpretation of the annunciation, the archangel Gabriel's announcement to Mary that she will soon bear the Christian messiah. Except that Mary looks to be receiving the announcement from an extraterrestrial vehicle, its message beaming down laser-like to reach her. It's the kind of light, so say proponents, consistent with those reported in modern-day alien abductions, presenting fodder for the theory that Mary was, well, abducted and artificially inseminated by aliens. 

Aert de Gelder, The Baptism of Christ (c. 1710)
Masolino da Panicale, The Miracle of the Snow (c. 1428–32)