The biggest differences between a wedding & a sit-down dinner at a nice restaurant is that at a wedding, all the food has to be served at once and the catering staff can only do one event per night.
All the food is served at once That mid-tier restaurant that you're thinking of, where dinner would reasonably cost $40-50/head including alcohol and tip. Let's say the restaurant can hold 40 or 50 people. Diners would come in over time and even at peak capacity, the kitchen only needs to be putting out ~15 dishes at the same time. At a wedding, however, all the food has to be ready immediately. In the kitchen this means either 1. more equipment (bigger roaster, bigger ovens, bigger storage -- they've also got transportation costs that a restaurant doesn't) or 2. more staff (to cook meals individually). It also means that you'll need way more waiters to bring all the food out. So, serving the food at once increases the staff requirements -- the labor costs should be 2 or 3 times what that ideal mid-tier restaurant has to bear.
The catering staff only does one event per night Once you've got all that staff, though, you only use it for a single peak. While a normal restaurant will have diners coming in over a period of many hours, the wedding caterer only gets one sitting. That means that the larger-than-average staff (compared to that ideal restaurant) has much lower utilization.
So, yes, caterers may be trying to gouge you on your wedding, but realistically they have a very different cost structure from the restaurants you're used to eating at.