Have you noticed that your waiter/ waitress always puts the question to you just after you've transferred a substantial forkful of victual into your mouth? You have haven't you? I have a theory.
I suggest that it's part of their front-of-house training. To indicate "Yes, it's fine" without generating an embarrassing food-overflow, you can either nod your head slowly or make a 'thumbs-up' gesture. Easy to do. To indicate that all is not satisfactory you will have to shake your head and finish your mastication of the mouthful of food before you are able to explain the reason for your dissatisfaction. Similarly for a 'thumbs-down' gesture. Unless you suspect you've been poisoned, you will usually opt for the affirmative option that involves far less hassle. And - of course - they know it!! An essential lesson for any aspiring waiting person. QED !!