If you sequentially attack a subpopulation (e.g., employees at a company, senior citizens) the eventually news about your attack will spread throughout that network (internal security alert, evening news+daily newspapers+AARP+...). If you attack the entire subpopulation in rapid succession over the course of days or even hours, educational countermeasures become much less effective.
Restaurants make or break on one or two nights in a month. A calculated social engineering attack like this could bring down hundreds of restaurants in a city, which would cause millions of dollars in lost taxes, and you see where this is going.
You could write a screen scraper to book online through the various booking systems, but each booking system probably has its own restrictions on how many accounts you can have and how often they can book. You skip all of these protections when you phone your reservation in (arguably, the restaurant staff should be enforcing these protections when they pick up the phone, but restaurant staff are often overworked and apathetic).