I'm sure it is at least partially just what you can get away with charging. Single burners are common for students and people with little room for anything lar

I'm sure it is at least partially just what you can get away with charging. Single burners are common for students and people with little room for anything larger, often very budget constrained. Cooktops are often being put in as a part of an already expensive kitchen refit/build. With that said, I'm sure there are other factors. Built-in hobs tend to need to be a lot thinner to fit in the available space, at least from what I have seen.