Even 100 years of aquarium breeding does not negate a few million years of survival of the fittest conditioning of evolution.
Many of the latent bacterial maladies are not acute, but show up as chronic infections at maturity.
Many oscars don't exhibit the ravages of being forced to live in abhorrent water parameters for a year or two, but at 2 or 3 years of age become scarred up with HITH from being crammed into a nitrate soup and hard water in an inadequate sized tank, and a lack luster water change routine..
You've only to walk into a LFS that accepts cast off oscars to see the damage done from hole-in-the-head, and other infections where water parameters didn't meet the evolutionary needs of a fishes natural immunity to certain bacteria that proliferate in non acceptable conditions.
And/or if one peruses the disease section filled with seemingly "mysterious" aliments espoused to have no rhyme or reason, but with investigation usually equated to adverse water parameters of one form, or another.
And not only oscars, but those many Geophagines, Uaru and other species from rivers like the Rio Negro, or Orinoco evolved to resist soft water bacteria, but not equip to high pH hard water phages in hard, high pH water.