I don’t think that’s usually how it works. A regular mask is good at “curbing the fanout” so to speak, but not so great at filtering incoming air. Taking your mask off while you eat is going to raise the risk for everybody around you—unless you use an N95 mask or similar equipment (and use it properly), what your mask does is mostly in fact protecting bystanders from asymptomatic COVID that you may have acquired by restricting outgoing airflow and virions within.
Now consider real life, where not everyone wears masks, and my guess is depending on where you are easily less than 10% of mask-wearers wear masks properly. They don’t wear masks over their nose, leave huge gaps between the mask and their face, take off their mask to sneeze or cough (I kid you not). They try to make it easier to breathe, and by extension defeat the point of wearing a mask.
Unsurprisingly, masks is the easiest and cheapest measure to be taken, compared to e.g. auditing and upgrading air circulation infrastructure.
Unsurprisingly, in many cultures people who think that the mask protects them and don’t know about asymptomatic COVID choose not to wear a mask because they bravely don’t care if they get sick.
Unsurprisingly, for the rest of us there’s very little point in being the one-in-ten person wearing a mask in what is a fluid exchange fest anyway.
Unsurprisingly, as a result people tend to not wear masks if they can get away with it, and restaurant owners and other people in charge of enforcing the measure on their territory would instruct staff to let things slip rather than alienate their patrons over a seemingly useless measure.
Meanwhile, masks are not free. Both in terms of money, and in terms of wellbeing. A properly worn mask necessarily obstructs breathing. One risks becoming infected by their own mask which accumulates virions in circulation (and if you do rotate your mask, an argument could be made that reduced exposure to viruses long-term may weaken the immune system—though I’m not sure if there were studies to support or refute that).
Nevertheless, a measure has been taken, and local policymakers may now rest with a false sense of security.