All of these methods are many many many times more expensive than just
running coax or fiber alongside the power line and using that for
data.

Or is there some sort of magic, yet cheap, shielding that they could
do that I'm just not aware of?



There's conduit and to a lesser extent BX wiring in the house, but
nobody's going to change
out the romex to get this shielding. And you still have all those
unshielded portable power
cords feeding table lamps, toasters, TV sets and such...