Quote~ short answer: At a time in history when only priests were allowed to read and interpret the Bible, where Faith was based on what was seen and heard, visual cues were important communication techniques: i.e., Satan’s Demons are real and they will get you. Nobody is worshiping the gargoyles so they are not idols. If, as a symbol of the Wrath of The Almighty, they scare demons, that’s a good thing. It’s a timesaver for St. Michael’s crew. Or an early-warning system, even.
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So if only the priests at that time, read and interpreted Scripture, then it was the priests that decided it was ok to adorn a “house of worship” with demonic gargoyles.
the priests were dealing with superstitious people building civilization from the ashes of the Dark Ages.
Churches had “heads” of saints that moved, too. Statues that bled, more pieces of the cross and Lances of Longinus than you could shake a stick at - all kinds of ‘visual aids’ to impress people with the power of The Word over pagan spells and conjuring.