Because the cows in the milk industry are mostly inside, the cows have to have calfs to get milk, so they get inseminated and when the calf is born its taken from its mother. They get explotied by humans all their lives, and is killed when they are about 4 years. Cows have a normal potential ifespan of 20 years. Also a lot of milk prevent the absorption of iron and contains hormones.
There is no disadvantage of using plant based milk in food or to drink. I cook a lot.