I get why restaurants use the cartoon-looking slop. If you don't particularly care that it signals that you put no effort into your menu or ad or that your restaurant's menu has the same shitty aesthetic as all other slop, fine. It's a cartoon and I could understand someone thinking it looks ok.
I absolutely do not get the uncanny-valley images. Those all look gross. Besides not actually being representative of the dish, the images have all sorts of weird texture issues, look too glossy, and are just unappetizing. I cannot understand why a restaurant owner would want to put up an ad that has food that looks straight up gross.
Professional food photography is expensive. This is a case of the free mediocre offer crowding out the much better but expensive option. Same way that free mail with ads crowded out email without ads.