They are resilient Tug but consider that when in the hatchery they are selected for different freshwater traits. Maybe earlier return, they don't select their mates, they may incubate in warmer water, may rear in warmer water, never have to deal with a freshet/flood, and so on. All of these traits are inheritable.

That is why, basically, Chambers Creek steelhead suck in the wild. They were selected for a radically different FW environment. Salmon are not so bad, but the hatchery still is different.

Remember, too, that the stocks did evolve to use strays, but it was likely only a small fraction of the run. If strays make up the majority, then the majority is not well-adapted to the system.