But hey! Let me qualify that a bit.... I mean if you're talking about infants, its a practice that's gone on since......well since some women either couldn't or wouldn't breast feed their own infant.
Many rich women in the past paid servants to 'wet nurse' their children so that they didn't have to bother with all that basic animalistic (and time consuming) stuff! For human infants - it's the best milk of all!
But I think you mean why don't we drink breast milk the way we drink other animal's milk .... That is after we grow up?
Well - the answer is we still can, and it would be better for us than, say cow's milk, but we don't! Why? Because we can't produce it in enough quantity. It would mean women expressing breast milk on a commercial scale! Imagine! No way!
Have a look at this video I found where they're asking people if they'd drink breast milk Ice-cream - I was surprised at the reaction - I wonder if you will be!!