The incentive is to produce tasty food that makes as much money as possible. Food is so cheap in the US that I doubt that was the limiting factor. Like, we make cheap candy that is 99% sugar.
If say soda were 10% more expensive, how many sales would be lost? Would the risk of losing sales justify reducing the amount of sweetener to retain those customers? Would Americans then recalibrate their taste buds to less sweet beverages?
FWIW, American packaged junk food like potato chips often has lower sodium today than it used to, so there's evidence that you can slowly adjust this stuff without losing sales.
After more than a year of Keto and not having had any bread, I ate some wheat bread again for the first time. Nothing sweet on top. Just bread (with butter on top). It tasted sweet! And it wasn't even your typical North American floppy toast white bread but just regular, proper wheat bread like you might eat in France in your neighborhood corner bakery (here they usually call it "artisan bread" now).
That sweetness went away really quickly after eating bread more regularly again but it surprised me nonetheless and makes your point on getting used to things.
Another NA example is cup cakes. There are cup cakes and then there are US cup cakes. I really don't like cup cakes at all. I avoid them. It's a tiny bit of cream with a ton of sugar. Just disgusting. They do the same with actual cakes. You can't buy a proper cake with cream based frosting and some tactful addition of sugar. It's a tiny amount of cream with a load of sugar on a tasteless body of (in many cases overly dry) sponge cake.
Take Subway "bread" as an example. It's so sugary it's classed as cake in Ireland.