Reason is the udder being not far away from the anus so literally shit happens. But there are procedures in the chain that reduce the risk. The milk is pasteurised and homogenised for this reason.
And this is exactly why i recommend raw milk to all my MAGA friends, hoping they will develop a natural resistance against being fed shit.
Maybe not smoking so much weed might help with this.
I quit smoking weed after I had been looking for my phone under the bed for half an hour - using the flashlight of my phone.