Restaurants MUST throw away food because it would look bad to have homeless people around the area, public image and all. I am against this wasting of food.
I've had friends collect waste food from restaurants for the homeless under the Food not Bombs banner in Dublin. After a couple of days the Guards (Irish police) put a stop to it. Quite a few restaurants and bakeries did donate, although many did not.
One way to solve this might be to institute some kind of cultural practice of largesse. That is, if it becomes disreputable to throw away food and a kind of hour of charity is created, that might help.
The devil is in the details, but this may be one way to go about it, even if not daily, even if not perfect.
There are cultures around the world where it is really, really bad to throw away food. People with extra food that won't be eaten as leftovers will offer it to less fortunate people who live or work in the neighborhood.
There's logistical issues, like: restaurants are busy and usually can't spare somebody to go find somebody to take excess, having excess is unpredictable so setting up a system is hard, having 8000 loaves of bread or whatever is too much (nobody wants to eat that much bread), etc.
Also if the restaurant/grocery/etc. is getting rid of food, it's usually in the neighborhood of not being suitable for consumption anymore. If it's not suitable for consumption, you can't really make a great case for then collecting it and giving it to somebody to eat, even if they're hungry and poor. So you need someone to have good food, care for it safely, but not want to use/sell it but still go through a lot of effort to get it to somebody who can use it.
There absolutely is some room in there for like "yeah we're getting rid of this because we don't want to sell it but it is still absolutely safe to consume", but it's just a difficult needle to thread in an area of society where there's never enough money or manpower.
N=1, but my girlfriend sells bread and when there are events she gladly gives a lot more than one slice because she knows otherwise it would be just thrown out.
Gathering and distribution for that would be complicated, have to keep all the gathered items food safe while you gather and then distribute. It's much simpler to operate a dedicated kitchen with donated/discounted supplies on many axis.
The son of a friend is with a group that collects unused restaurant food and distributes it. I think it's in the Davis CA area. If you want more details i can find out, but I'm guessing you just want to know if they exist.
I am not sure, definitely unheard of in my country. If you know of a country that does this (or maybe a state in the US, or just a city somewhere) I would like to know for the sake of restoring some faith in humanity.