Why is a cup of coffee not 6 oz.? According to Google, a cup is 8 oz.

According to Wikipedia, a cup of coffee as measured on the coffee maker is only 5.07 oz.

Every brand of coffee I buy shows directions for 1 tbsp. per 6 oz..

So, why is a cup of coffee only 5.07 oz. when nobody (that I know of) uses a cup that small? In fact, in most establishments, you can't even buy a "small" cup of coffee that is less than 8 oz. 

Is this just a conspiracy so that the coffee maker companies can exagerate their capacity while the coffee companies dupe people into using too much coffee per cup or is there another reason? 

Why not just make it 6 oz.?
Do not blame the coffee makers for what has akways been an ambiguous meassure - or rather, not a unit for meassuring at all. Only in some areas of the world a "cup" has evolved into a fixed unit.

First and foremost, a cup is a drinking vessel for usually hot beverages and usually with a handle.

How big a cup is, is primarily a cultural thing. To showcase two extremes:

An espresso cup 

(source)

vs. a jumbo coffee mug .

(source) 

I am from Germany. We typically use [ml] for volumetric meassurements. Whenever I come across a recipe quoting "cups" I start my research: Where did it originate? Is it from the US? I use 245 ml. From current Germany? Probably 200 ml. From my Granny's time? 150 to 180 ml probably fits the bill.

Similar rues apply for coffee, from a small espresso or mokka cup in southern Europe or the Arab world to a mug of filtered coffee in the US. 

There seems to be one correlation though - the "stronger" (=more concetrated) a coffe is customarily prepared, the smaller the cup. The amount of ground coffee per "serving unit" seems surprisingly constant for the various preparation styles - about a heaping teaspoon per "cup". The rest is a matter of taste.