Also worth noting: "[Michael's] churches were often located in elevated spots", says the page on San Michele Arcangelo, Perugia.
The obvious next step here is gathering a database of every spot claimed to be sacred to Michael, plotting them on a map, and seeing if this particular set of seven places leaps out of the data. But that sure sounds like work.
(Well, that's the obvious next data scientist step, there's also the obvious next step for the magician or priest, which is to go to or create a sacred space suitable for summoning archangels, call down Michael, and say "hey thanks for coming, so what's up with this line we call your sword?".)