Why would president george washington warn the country to remain neutral, avoid debt, avoid sectionalism, and avoid political parties?

Because there were bigger, stronger, older nations that would
have loved to get their hands on that rich territory that the United
States covered ... France, England, Spain, etc.  Washington knew
that the young USA could only survive as a nation if the people
remained united, and all those things ... taking sides in international
conflict (not remaining neutral), national debt, sectionalism (north
against south, east against west, old states against new states),
political parties (Whigs against Tories, Republicans against
Democrats), are things that could set groups of the people
against each other.