Actors on TV or on film are seldom filmed placing food
in their mouths, then chewing and swallowing. If they are, it is often a signifier that the character is uncouth (or at best blunt and direct).
Usually the edit shows people preparing or ordering food, in front of food, holding food, talking over food, complimenting the food that they have just eaten, but not actually eating.
A typical fast food ad must try to sell food, so it
follows the pattern of wacky antics and loving close-ups of the product, followed by the character moving in for a big happy bite. And as his mouth closes on the burger (no actual teeth or biting shown), or just after she discreetly pops the chocolate into her mouth, we cut to the logo before we have to deal with all the chewing and swallowing.
We, as human social animals, have an instinctual desire to eat with friends and family, but we also have an slight innate revulsion to other people eating in public. So we tend not to eat front of strangers or people who are not themselves eating. As Ikura notes, sharing food is a social bond.
Aside from that, the process of multiple takes means that the actors can't take a bite each time the scene is shot.
Most British TV ads can be divided into one of two equally sized groups: ads for fast food and sweets; and ads for beauty products to repair the damage done by the fast food.
Of course, you could shun both and still come out even, but then you wouldn't be a good consumer.