The behavior of cats stepping on milk can be traced back to the period of nursing cats. When the kitten snuggles up in its mother's arms and suckles, it sucks hard and presses its mother's stomach with two small paws, so that it can secrete more milk.
When the cat grows up, although it doesn't need breastfeeding, it feels warm and safe to snuggle up to its owner. At this time, their memories of the cat-sucking period came out again, so they began to step on their masters contentedly.