Don't know why - something like plain yogurt lets the bugs settle down and behave. Great for ulcer patients. Not so good on the waist. Although, if milk is on your eating list, this can work too. I have been known to eat an entire batch of tapioca pudding in lieu of dinner when I haven't felt good.
Custards never get to cool.
Or not often anyway. Nothing wrong with starting with an instant version. I use Watkins vanilla and always add extra vanilla. I've learned to settle for powdered skim milk and egg substitutes. You cholesterol levels and waistline will dictate.