Why are large cities often thought of as states within a state?

Answer 1

A fitting example of this is the state of New York. In 2016, Hillary Clinton won the state by almost 2 million votes. However, Clinton won only 17 out of 62 counties. How can this be? Most of the votes for Clinton came from New York City, obviously the biggest city in New York. No matter how many votes are cast for a Republican, a Democrat will more than likely win because of NYC. There is always talk in upstate New York about separating from NYC and Long Island because of this fact, as well as the fact that a majority of money goes to NYC interests.