It's not the same (tasting) stuff in each bottle; everyone adds their little touch to it - a different mash bill, different wood for aging, different types of woods, different blends, etc.
There is some labor & R&D here that can't strictly be - ahem - distilled to dollars and cents.
I'm not sure MGP resellers control the mash bill; rather, MGP sells a variety of different whiskeys, so they presumably get to pick from a menu.
The aging process however makes a huge difference; it accounts for probably most of the flavor of the whiskey. So there's definitely potential for bottlers to add value. Unfortunately, most of them probably don't, because the things that add value --- sourcing quality large barrels instead of small casks, aging carefully for a long time --- add cost and delay time to market.
Don't forget blending. Most "famous" names in whiskey are there because they know which barrels to blend together. MGP sells quality distillates in a wide variety of mash bills. There is no reason a good reseller couldn't create great whiskey blending and aging their product. Do any of their current resellers? Hard to say, but contract distilling is not new or rare so I'm not sure what the fuss is about.
There is some labor & R&D here that can't strictly be - ahem - distilled to dollars and cents.