The "Columbian Exchange" line of fantasy criticism catching on so much is evidence that most people do not actually think about fiction very well. Tolkien can get tweaked for it because he was explicitly writing a pre-history of Europe, but no, it is not in fact a universal narrative law that, in a fantasy world with multiple continents, potatoes, tomatoes, and maize will always be native to one continent and apples, brassica cultivars, and wheat will be native to another, why the hell would it be.
Of course, you could say that about other cultures-bought-in-bulk fantasy stereotypes. No reason that Thrygvar Wulthenstaal can't be a bearded, turban-wearing man from an agrarian culture who mastered the macuahuitl to drive away the leopards and jackals that threaten his freshwater fish farm. But readers and writers alike don't seem to want that.
Allow me to propose as a tongue-in-cheek replacement the "British Empire" line of fantasy criticism, for something else copying modern effects without their cause.
Through history, the vast majority of people and cultures have only opposed slavery when it happens to the ingroup. "But what if you were the slave?" Well that line of argument was thoroughly tested with Ayuba Solomon Diallo, who was first a wealthy slaver in Senegal, and then he was captured and sold into slavery and shipped off to America to work on a plantation, and then he managed to get his freedom and come home, and he went back to slaving again. The actual experience of being enslaved in the very bad way didn't convince him slavery was wrong, only that it should happen to his enemies. The idea of mass convincing people by mere argument is absurd.
Abolitionism, in the sense of broadly generalized opposition to slavery, was historically extremely rare. There have been many lords and rulers who rescued friends and neighbors from slavery out of personal interest, or manumitted some slaves to show off their wealth and charity, or declared Jubilee for a great celebration, but they did not reason that they should end slavery as such. The fact that abolitionism is widespread in the present is a bizarre historical fluke.
Abolitionism popped up only about three times and places in history independently: First in medieval Western Europe around people like Queen Balthild, which died out when the Renaissance brought slavery back because all the cool people (Romans) had been doing it. Second in medieval Korea, which didn't spread beyond the border of Korea, and I say "border" in singular because their sole neighbor for much of that time was China. Sometimes they were adjacent to a mongol horde too, but those didn't care much for borders anyway. ;-)
Third in Renaissance/Industrial Age Britain, which is the one responsible for pretty much all other abolitionism in the world through the power and prestige of the British Empire and its influence through the colonies. Sometimes a country like Japan would decide to westernize by copying everything from Britain including the ban on slavery, sometimes the Brits forced it on other people at gunpoint. The general course of the latter was that the British would sail the Royal Navy over to Mwambaland and say "Hey, you're going to abolish slavery now" and the King of Mwamba would say "Fuck no I'm not, I need those slaves for my mines" and then the Brits would kill the King of Mwamba and most of his army and family tree and find some grand-nephew to put on the Throne of Mwamba as a puppet ruler and tell him "You're going to write a constitution abolishing slavery now, also we get the tariff revenue from your ports".
[insert nuance and caveats because I'm summarizing centuries to a paragraph, history has a lot of detail that you can read about in longer books.]
So if you see widespread abolitionism in fantasy, you might ask: "when was the British Empire here?" :^D
