Lets use engine additives as an example. Drive to walmart and head to tire and lube department, or head to autozone or advance auto parts. Look at all those additives! Not one manufacturer has ever printed "oh yeah, toss this in with your oil and you will get 300 mpg." Ask an engineer what he thinks about putting slick50 in an engine and he'll likely bitchslap you. At best these provide very short term advantages (like fuel injector cleaner in the gas tank) and at worst, they can destroy a catalytic convertor (Some honda civics really hate additives) or kill an engine (using a motor oil flush in an old engine that otherwise runs just fine). And yet, these additives fly off the shelves because salesmen throw a pretty display up that is supposed to somehow prove using Lucas is better than just plain oil.
It's about marketing, not actual results. If you can trick the customer, and have enough variables thrown into the actual usage of the product to cover your ass, you win.