Because "the fake thing" probably wouldn't sell a lot of Coke. Also, to subtlely suggest Coca Cola to be separate and superior to other products with a similar name, ie Pepsi Cola and RC Cola.
Coke
Examples of a slogan include 'The happiest place on Earth.' used by Disneyland and 'It's the real thing.' used by Coca Cola. A slogan is a memorable phrase used to express a purpose or an idea.
You Know When It's Real or Chewy and Moist.
Let me manifest, if you want to live, digest! And yes, that's not a real slogan.
The real coffee experts.
If Xerox Corporation can be said to have a business slogan, it is probably "Ready for real business."
TONKA
I'm lovin' it You know when it's real
The slogan "Building India" belongs to a company called DLF, which is the largest real estate company in India.
Dont give in, we want the real you to win!
Gotland --FreedomFalling HI2
Where homes click! But, you have to brand yourself as a .com company.