They've developed channel power, probably not on the same level as Coke, but something like that.
Once you have your place 'in the system' etc. you have incredible leverage.
The chocolate bars at my convenience store are the same, boring 20 variations for my entire life. They are not 'the best products' - they are just the products that will work through the system. It'd take a decade of consumer awareness to put something on par with 'Coffee Crisp' and then you have to get all of the gears of distribution to go along with it.
So what we end up up with in chocolate - and any number of other goods - is kind of a 'channel equilibrium' where products establish channel dominance (which requires scale), and then inch forward ever so slightly.
To see a 'new' product get to that level ... take a look at 'Swiffer' (cleaner) - basically, that's an institutional effort by P&G to 'disrupt' their own categories a bit and they've spent zillions marketing that. It's a trivial little thing, not much in the way of innovation, but when were talking consumer products at that scale, it never was about 'innovation'.
These companies do buy brands and scale them but even then brands meet their limit. 'Ben and Jerries' they can probably 10-50x in size ... but at that price point they can only go so far with the category. Odwalla ... sadly had to shut down.