Why? So the tribal natives don't get wiped out and superseded by industrial-age settlers? Is that the outcome you'd have preferred in our world? I mean, I don't like the idea of genocidal total war or slavery, but they got you and me to the point where we're arguing on the Internet. And our genocide was less justified (in that native Americans didn't, as a group, declare "blood feud", i.e. the intent to perform a genocidal total war of their own, on American colonists), and our slavery of a much crueler sort, than those depicted as used of the Seanchan in WOT.

Do you think Shannow in Cairhien, 1050 NE, subject of the Empress, may she live forever, now enjoying the fruits of the industrial revolution and nine of whose relatives a few generations back lost their lives to crazed Aiel bloodlust, would find Aviendha's vision horrifying? How about Shannow's grandparent in independent Cairhien in 1000 NE, whose entire family just one generation back was butchered for defending their nation from Aiel invaders?Somebody's going to do it. Or try. For the same reason as it happened in reality: because they think they can. Whether it's the monstrous, enslaving Seanchan or the monstrous, repressive Tairens or the monstrous, enslaving Aiel who live for war (fat chance) or the benevolent, godly Andorans makes no difference to the dead and probably not much in the end to the living.
You don't have much of an imagination.