Because "American cheese" and American cheese product is processed. it varies in quality GREATLY among brands, and sources.
you can get American Deli sliced here, so it will be fresh and usually far greater quality then Kraft, etc.
Krafts and Velveetas formulas are super melty though. Which does make it ideal for certain foods, in the same way cheeze whiz is poison, unless its on a Philly Cheesesteak.
Swiss, vermont cheddar, monterey jack, Colby, Bergenost, Cream cheese are all American based cheeses.
Cheddar originated in England of course. But was quickly brought over, and primarily used, and US forms of it.
"american cheese product" is actually just processed cheddar.