Some families use different types of milk (goat, soy, cow) and I don't want to keep all different kinds stocked.
My family drinks a very small amount of cows milk so we don't go through much.
I have also found that the amount of milk one child drinks compared to another can vary a lot... Some of my dcks drink 8-12oz per day... My own 19m son drinks maybe 4oz per day, he only recently stopped nursing so never really drank much cow's milk.
It seems like different families have different expectations as to how much milk a child should be drinking so it's easier for me if the parents send their own.
Plus, one less thing for me to spend my $$ on.