By definition, the conclusion of any deductive argument follows directly from the premises. For example, consider the following famous syllogism:
Premise 1 - All men are mortal.
Premise 2 - Socrates is a man.
Conclusion - Therefore, Socrates is mortal.
Notice that the conclusion doesn't state anything new; it is just a restatement of information contained in the two premises. All of modern mathematics is based on this type of reasoning. Because of this, mathematics should be obvious since it just restates things that we already know.
Yet mathematics is not obvious, as many mathematical discoveries are surprising. How can a style of argument that just restates the premises be considered to increase our store of knowledge in any non-trivial way?