Ketchup is basically sugar. With 2 of the first 3 ingredients in ketchup being high fructose corn syrup and sugar, Ketchup fits America’s need for sweet products, sweet/savory combinations. Ketchup has no fat in it and is basically entirely sugar. This produces a drastically different hormonal effect in the body when consumed (sugar high, followed by huge insulin spike, followed by lethargy).
Mayo is essentially entirely fat with the main components being egg yolk and different oils (depending on the recipe). Strangely, mayo takes on different colors in America vs. Argentina. In the U.S. mayo is starkly white. This strikes me as odd for a food that is based on the yellow egg yolk because in Argentina, it is much more yellow.
The 100% fat content of mayo satiates the appetite and gives people a full feeling. This hormonal effect of satiation is much different than the roller coaster sugar high effect of ketchup.
Not to mention that the flavor of mayo is not sweet at all. It is entirely savory, another thing that illustrates the two countries’ preferences for sugar vs. fat, sugar high vs fat satiation, ketchup vs mayonnaise.
The odd thing about the practice of serving the two condiments is that ketchup in the U.S. is omnipresent. It’s on every table in every restaurant. ANYTIME fries are served, ketchup is there, even in high end establishments. Mayo would be odd in that circumstance.