TITLE: Why are so many of the oldest unsolved problems in mathematics about number theory?
QUESTION [30 upvotes]: Stillwell mentions in his book, Mathematics and its History that: 

Most of the really old unsolved problems in mathematics, in fact, are
  simple questions  about the natural numbers...

Have people tried to understand what it is about the "topology" (figuratively speaking) of number theory that makes open problems from it notoriously difficult to solve?

REPLY [6 votes]: I had a math professor (elementary number theory) who jokingly hypothesized this was because we live in a continuous world, so proofs are more obvious in analysis than number theory.  "Intermediate value theorem?  It's continuous function!  You drew it without lifting the pencil off the paper.  Who thought up that?"