That and nobody wants to eat a meal 40 times to get 40 takes.
BUT if you eat the food in one shot you need to eat it in all the shots for continuity, so you can edit it together. Get ready to start barfing after 40 big bites of the same damn thing.
If you look closely, you'll also see the coffee/tea cups actors sip from are usually empty. Can't afford the risk of accidentally spilling liquid on the costume and delaying the shoot.
Sometimes they are colored water, so you cannot drink but it still looks like a cocktail. Or at least that's how it was on the few movie sets I've been at.
If I were a prop-master (is that what it is called?) I've always thought that I'd just have a bag of plaster of paris handy. Then 30 minutes before going on set just dump some in the prop-cup with some water.
Sets quickly, density is about the same, physics of the cup should look convincing. Probably best for disposable cups though.
I guess its subconscious - they know they are not going to actually drink it, they focus their mind on other aspects of acting, so this part leaves them not faking it well.
If you see it once, you can't stop noticing it elsewhere afterwards, beware.
I want to believe the gagh is real.
Except maybe Brad Pitt (see Ocean's Eleven).