The reason is simple, that is, people are not cows.
Although this sentence looks the same as the one given, the truth lies in it.
There are many differences between people and cows in this matter. Man has only one stomach, and cattle belong to ruminants. They have four stomachs, namely rumen, honeycomb stomach, bicuspid stomach and abomasum. Rumen is the largest, accounting for 80% of the stomach volume, and it is the main place for food digestion.
When the rumen is mature, it is not afraid to eat grass. Rumen is a natural fermentation tank, in which many harmful microorganisms are killed.
So cows are not afraid of this at all.
Humans are actually not so afraid, and some people will drink raw milk. When the ph value of human gastric acid is lower than 3, some bacteria can be killed, but not all.