Because no one ever eats foods they ate when they are 5...
I want to know who started this "ketchup is for kids" nonsense. Slathering massive amounts of ketchup on everything is for kids, using it judiciously is fine. I love french fries with ketchup and sometimes a little hot sauce (tabasco) as well.
It's true, though. the reasoning is that sweetness/sugar is the most overwhelming taste that you experience. Ketchup is basically just sweet, which is what kids prefer to put on anything. Not so much because they prefer sweet, but because it actually takes 12 +/- years for the human tastebuds to actually mature.
The younger you are, the more dominant is your sweet profile, and everything else is very off-putting. Sour and bitter are especially bad (inherited signal from our tree-swinging, cave-humping ancestors that these tastes = poison).
So, ketchup is a ubiquitous condiment for children because there is a strong, developmental imperative to reject most other tastes. Typically, adults move away from this because as you age, you tend to appreciate other tastes and what never changes is that "sweet" will dominate everything in your palate.
ergo--malt vinegar is for French fries/potatoes. NEVER ketchup (...unless you are 12).
Don't hate the message--hate biology?
Of course, there are those that certainly prefer sweet over everything else. Either they just like it, they are wired that way, or never allowed themselves to try different things, because those childhood memories of "food taste bad!" stick with them, not realizing that these are the memories of an unrefined palate. For what it's worth, sweet is my least favorite taste.