Let’s not put all the blame on the director. Someone had to write that scene. I’ll break down my view on the matter by each element.
Glass container – It shatters nicely. Let’s be honest – we love the sound of shattered glass. Also, a glass container is always a nice metaphor for the human condition/body.
Milk – I haven’t seen milk used all too often, but I would assume they use it for its thickness. Playing on the glass/human metaphor, milk is closer to blood than water. Other liquids would be better, but milk is more common. It is also more visible than water when it splatters.
Slow Motion – Because having a glass of milk fall in real time, would be too damn quick. Which is great for horror flicks. The action & sound is sudden, no time to think. For dramatic movies, slowing down time gives us a moment to process the situation & maybe get inside the characters head & try to imagine what they are thinking before the glass shatters.
This is just what comes to my mind. Every viewer will interpret these meanings/purposes differently. I agree that it is a bit overdone, but when used in the right place/right time, it is quite effective.