The other preferred view on the matter is that they take based on hostility. When they view a tiny fish in their location they get perturbed and also they go after it. With swing flies the fly will appear like its taking off the area because of the presence of a larger predator and also that's when the steelhead strikes. This is the thought behind Spey fishing.
Nobody actually knows why steelhead take flies. Like salmon, once steelhead enter fresh water they avoid proactively feeding as well as simply rely upon body fat storage to make it up the river, generate, then return to the ocean where they will gorge themselves and offset the long fast in the fresh water. So it's type of baffling that we fish relying on replicas of insects and lure fish that they do not even eat in the sea yet are respected to the streams.