House flies are attracted to the sugary 'honey dew ' excreted by the aphids. That's easy to figure out. I would be wondering, however, where all the flies are coming from.

"Millions" of house flies are close to a million too many! Where do you think they might be breeding? They're pretty specific as to where they will lay their eggs; those maggots have to feed on some kind of sloppy decaying stuff, manure, etc.

Well, we do have feed lots around here, but a friend of mine who lives 35 miles from me has the same thing going on