The only difference is branding and price point. You're right in noticing that the ingredients are really no different between a food you buy at the grocery store versus Science Diet or Medical. Unfortunately, even though the second two are pricey, the quality is no better than Whiskas.
No animal should be eating wheat, corn, soy or chicken/meat by-products.
Google some of these brands:
Innova (EVO line)
Blue Buffalo (Wilderness)
You'll notice that they contain very few, if any, grains and REAL meat is the #1 ingredient. This is what makes a good pet food!
If you use the "Store Locator" function on any of their websites you'll notice that these brands are found in pet supply stores, not grocery or large chain stores. You have to go looking to find good food!
There is a user on here (sugarcatmom
) who is very well-researched in cat food specifically, and I'm sure she'll notice your thread later on and provide more information.
Chase (Friendly Resident Wigglebum) - Border Collie/Lab/Shepherd X
Kailey (Misunderstood Gentle Beauty) - GSD & foster failure #1
Rupert (Gold-Medal Winner of the 3 a.m. Kitty Destruction Olympics
Heidi - RIP my sweet baby girl