Humans are good at pulling nutrients from all sorts of sources but those sources have to actually contain the nutrients in the first place, we don’t have some magic ability to just eat one thing with no supplementation and get all our nutrients.

Dogs are omnivores.

Supplements are already in the livestock (that we feed the cats) feed and animal based cat food. Yes it’s harder to get most cats to take a pill than a human adult, but that really isn’t necessary it can just be put in the food itself, and it is.