This seemed a question right for the straight dope…
Cats are notoriously finicky eaters (as all cat food companies like to remind cat owners), and none of the cats I’ve met in my life were about to disprove them.
However, just about any “cat treat” is irresistible to just about any cat.
The ingredients do not seem different from the food itself.
What’s the trick?
And why don’t the cat food manufacturers simply make cat food that tastes like cat treats?
I have two cats.
One cat only likes the crunchy-granola cat treats you can get at Trader Joe’s. The Pounce and whatnot, he turns up his nose at. So it just goes to show you, you can’t pigeonhole your kitty audience too greatly. 
The other cat won’t eat cat treats at all, of any variety. She gets this weird look like, “What the hell are you trying to do? Feed me ** cat food?**” and waltzes off.
Her cat treats are in the pantry, and come in the bag with the “Ruffles” logo on it. No lie. 
She absolutely will pester you to no end to get a potato chip.
My cat also is immune to the charms of every single cat treat we’ve tried. However, he’ll ride a unicycle and sing Wagner for Gerber Strained Turkey.
Probably the same reason a nice salad can’t be replaced with a Snickers.
Same here. My cat is indifferent to treats, but a pretzel “Nibbler”? You do NOT want to grab a handful without making sure you give her one too. I believe she would eat my eyeballs in my sleep, if I didn’t share.
Ditto our cats. One of the girls will only eat crunchy, poultry-flavored treats that come in a foil-wrapped pouch, and even then she will only eat them if she has to “chase” them first (e.g., if we throw them down the hall or under some furniture). The other girl, our princess, will eat any flavor and consistency of treat out of any container, but only if they are fed to her out of hand, or placed upon her favorite velvet pillow. (Yes, our cats are very spoiled.)
And Owen, the boy? He’ll eat anything, period, but he especially likes whatever we humans are eating. (Right now he’s trying to get his grubby little mitts in my bowl of Frankenberry cereal.) This cat’s already jumped up on the kitchen counter and run off with a hunk of ham that weighed more than he does. He even ate through the foil.
Our cat will definitely eat most treats, even when he’s not interested in his regular food. There are a few he doesn’t care for, but as long as it’s fish or tuna flavor, they get gobbled up.
My guess is that treats have a stronger smell than regular cat food.
My cat will eat some cat treats, but not others. Specifically, he will eat the crunchy Tartar Control treats (no matter what brand or flavor) but refuses to eat any brand or flavor of soft Hairball Prevention treats (unless you put them in the food bowl with his catfood and then do not refill the bowl until it’s empty–if you do refil it, he eats the catfood and leaves the treats).
Obviously, the acceptance of cat treats isn’t as universal as the OP believes.
My cats will eat normal treats, but won’t touch the Hairball Prevention treats either.