After weaning, feline lactase activity typically drops, and cow milk's high lactose load commonly causes bloating and diarrhea, the classic lactose intolerance picture.
Goat milk has slightly less lactose and smaller fat globules so some cats tolerate it better, but it is not lactose-free; pet formulas are usually further adjusted.
Healthy adult cats do not need milk as nutrition; for hydration, plain water or wet food works better, and milk should stay an occasional small treat.
Orphan kittens need species-appropriate milk replacer rather than cow or goat milk; follow veterinary guidance on mixing and temperature to avoid severe diarrhea.