Because humans can't digest the protein of cows milk.
This is the first time i have ever heard that humans cannot digest cows milk.
I quit cereal & milk a few yrs ago but still put some in my coffee or eat some cheese & occasionally some ice cream( unless I am mistaken, these last 2 are also derived from cows milk).
I have also noticed that Ralphs market here in LA sells cows milk. Do you have any idea why they would be selling it if it is unfit for human consumption??