"Why is it called cat food and dog food instead of cat feed and dog feed?"
I guess there are a lot of oddities in any language.
Like "How are you doing?" as a greeting. How are you doing what??? (In German it's "Wie geht's?" Which means "How is it going with you?" - How is WHAT going with me?)
"Take care" as a parting statement. Take care of what?
"Como se llama Usted?" meaning "What do you call yourself" for "What's your name?" in Spanish. What I call myslef isn't important. Its what other people call me.
Languages can be weird.
Why is it called cat food and dog food instead of cat feed and dog feed?"
It is called food when associated with humans, feed when associated with animals.