I find it impossible to believe that this "sameness" is what people really want. I suspect candy vendors have specifically conditioned the candy market in much the same way as the film industry has conditioned audiences into go to see mass-market films-- endless re-hashes of the same stuff, endless sequels, formulaic plots, god forbid an edgy wildly different film gets out every once in a while.
Things are different in most European and Asian countries where you can easily find a mind-blowing variety of candies that are truly distinct from each other.
That reminds me, there's a fun book of essays, Candy Freak (https://www.amazon.com/Candyfreak-Journey-through-Chocolate-...) which is all about the vanishing fringe candy makers in the USA.
Uhm, in the US it's pretty easy to find boutique candy stores with a very wide variety. Even in grocery stores, the candy isle has quite a bit to choose from.
Don't conflate what's presented in the checkout rack, for impulse purchases, with the wider variety of what's available.
Once you try proper chocolate (I recommend Winans', http://www.winanscoralville.com/), a grocery-store candy bar becomes inedible. I don't want them in my mouth. Its that different.
Central Asia has pretty much only Mars products.
1 - https://en.wikipedia.org/wiki/Mars_(chocolate_bar)#United_St...