Whenever people end an argument with the phrase, "it's as simple as that" - it's almost always not so simple.  It's usually the case that the person just doesn't have any additional arguments to make.  It might also be that people think saying "it's as simple as that" makes them sound more convincing.  When I hear this phrase, I'm usually less convinced then I would otherwise be.  I think the people who use "it's as simple as that" also like to use multiple exclamation points and capital letters for emphasis when they write.  Like they're trying to sell me a Snuggie, a Sham-wow, or a "sure thing" investment system.