group dinners why does restaurant charge more for less?

I am planning a dinner for 18 up in the California wine country. The date is for a Saturday night in early spring, so I am planning 8-9 weeks ahead. I am looking only at restaurants that have an appropriate sized table or seating area. A few require room rental or a premium to seat this many guests. I spoke with a few restaurants already and almost all require a prix fixed menu. The prices are well above the listed menu prices.

I can understand room rental if I require a separate space, but for the few restaurants with chef's kitchen tables, or large tables that are just rearranged, why the big$$ premium? I think my preference would be to be in the main dining area if it accomodates a large party.

If my menu is restricted to the prix fix dinner, why is the per person charge well above the separate items on the menu?

And while I greatly value the service of a staff, and I generously reward accordingly, my gratuity is set at 20% on both food and beverage before I enter the door.

Do any event planners, or patrons, know or consider the above just a matter of course?

Thanks in advance for your input.

goldparamount in Not About Food