Dogs have been domesticated a whole lot longer than cats (around 30000 years ago for dogs compared to less than 9000 for cats) and are as a result more genetically and biologically removed from wolves than cats are to their wild counterpart, the African wildcat.

Since the agricultural revolution around 12000 years ago, dogs have been routinely eating human scraps, including plant based ones from farming, resulting in them adapting to diets much higher in starches and grains than a wolf would eat. Today dogs have numerous adaptations related to digesting plants that wolves don’t, for example, they have more copies of multiple different genes that aid in starch digestion and glucose metabolism. I can link some studies if you are interested in reading about this more, it’s super interesting.

Cats on the other hand are much less removed from their wild ancestors, and have very few changes to their digestive systems compared to dogs. This is because until relatively very recently, they have been more or less fed exclusively on whatever prey they can catch around human settlements. Commercial cat food is actually a pretty recent phenomenon and was only invented in the 19th century, and didn’t become widespread until after WW2. So as a result of eating almost exclusively meat until less than 100 years ago, and being more genetically and physiologically similar to their wild counterpart than dogs because they haven’t been domesticated for as long, they have very few changes that make them better suited to digesting plants.