Interesting question, and I get it a lot.
The word I get from restaurateurs I have spoken to about this is that 50-70% of the bread they put on the table is not eaten. Quality restaurants know that that bread has to be thrown away (once something has been on a customer's table, it cannot be reused — or at least it's not supposed to be…).
They blame, (1) the plethora of fad diets that have made people fear carbohydrates, along with, (2) those who decide they are not going to eat gluten for no stated reason other than being seen as “healthy” (no truly scientific basis for any of that, except of course in the event of an actual diagnosed medical allergy or sensitivity due to celiacs disease).
Many restaurants (Matt Haley's SoDelConcepts restaurants and Touch of Italy, for example) bake their own bread so they can control not only the cost (in-house bread doesn't have to be delivered) and the amount they serve/make in the short run. Other restaurants charge a very small amount for their bread so only the people who want it will order it. An example of this is the cornbread at Po'Boys Creole & Fresh Catch in Milton.
Quality restaurants survive on making you happy, but many chefs and owners are frustrated with the hundreds of dollars they throw in the trash. Some will pass the savings onto the customer by controlling menu prices, others will simply enjoy the few hundredths of a percent increase in profit. Either way, it's better than throwing untouched food in the trash.