To begin with there's no guarantee it's actually real food..it could be a prop.
Secondly, filming takes time and several takes so any actual food is likely to be cold and unpalatable by the end of shooting.
Eating on film can be a trial for actors as they often have to repeat the same scene over and over again for multiple takes.
Quite often what you perceive as eating is them moving food towards their mouth, they're usually stopping to continue dialogue. Only in rare events will you see food actually consumed..and then the above points apply.
That's not to say that actors don't eat it afterwards but that will depend on the quality of the food (and craft services are usually pretty good).
An example is Paris Can Wait..where the actress, Diane Lane, explained...
"I was overwhelmed by all the varieties of cheese that I was exposed to, and the vast regions of wine," she tells Bon Appétit. "I would wrap up whatever was on set and put it in my purse to take it home and eat it."
And although Lane admits that she's "not above spitting the food out" during a long day of eating scenes, "that definitely didn't happen on this film—the food was too good." But it is not as simple as enjoying a few bites of food while you're in the middle of a scene. Everything is about continuity so the film can ultimately be edited together in post-production.
"I'll start to notice if there's a ring forming under the glass or if it's sweating and it shouldn't be, or if the ice is melted, or if the glass is half empty versus how it was. You have to maintain things, because you can't go back two bites ago," Lane explains.
Lane has another big pet peeve about food scenes.
"I can't stand it when people really never put any food in their mouths in a scene," she says with an exasperated sigh. "Timing when you're gonna speak while you're eating is tricky, but I prefer it when you can dive in and mean it. Some actors chicken out."