George Washington retired at the end of a career in public service that was a lot longer than his 8 years as President. Between his plantation and his land speculation business, he was a commander in two separate wars and a leader of the country between four different governments and congresses from the Virginia House of Burgesses to the new United States. The short answer is, he already wanted to retire, long before becoming President, but the country kept demanding his leadership.

I wrote a horribly long answer here, which is in 3 parts—his career before & during the Revolution, after the Revolution, and as President. You'll have to forgive me if it includes too much information that does not concern Washington, but I also think the making of power and the larger transitions of power in early America are worth discussing even if they do not directly involve him.

** Before & During the Revolution.

Washington retired 3 separate times. He spent his early 20's in the service of the Virginia Regiment, fighting in the French and Indian War (1752—1758), and "retired" in the sense that he no longer sought a career in the British Army.

He then made his riches as a land speculator. For those poor tenant farmers on his thousands of acres of land, he was an absentee landlord. He married into considerable wealth, was now in the possession of a few hundred slaves and managed a large plantation. Because of his status as a prominent landowner and military hero, he was also active in the Virginia legislature, supplying voters with beer and brandy. In the 1760's, he was critical of British policies. In the 1770's, he served in the Continental Congress.

Then, he spent his 40's in the service of the Revolutionary Army, as the commanding general in a war that lasted 7 years (1775—1781). With the treaty in 1783, Washington resigned as commander-in-chief, retiring for a 2nd time. King George III commended Washington when he heard that he would resign his commission, leaving power in the hands of a weak Congress. The famous & apocryphal quote is often attributed to him, "If he does that, he will be the greatest man in the world."

Again, Washington returned to life on his plantation, and effectively declined to carve out any position of power for himself in the new independent country.

** After the Revolution.

To finance the Revolution, state governments spent $114 million, the national government spent $37 million. Now both were in debt. The IOU's paid out to soldiers (bonds issued by the Continental Congress) had little to no value and were bought up by speculators. The economy was in recession. Revolutionary soldiers were not getting paid, and many of them lived in poverty.

In 1783, Revolutionary Army officers in New York were on the brink of rebellion, in what is called the Newburgh Conspiracy. After General Washington gave a prepared statement, he could still read the resentment on their faces. But then, in a famous speech to his fellow officers, he fumbled around in his pocket for a letter he had written and pulled out a pair of glasses few people had seen him wear. He told them, "Gentlemen, you will permit me to put on my spectacles, for I have not only grown gray but almost blind in the service of my country." They were reminded of the sacrifices they had made, some of the officers were moved to tears, and the conspiracy fell through.

This is the backdrop of Washington resigning his commission. The national government was still in a weak place during the 1780's (often called the Critical Period), and the question of who would rule the country was still was open to a wide range of possibilities.

The weak national government under the Articles of Confederation was showing its defects. It had no power to collect taxes, no way to raise revenue, no way to pay its soldiers and no power to put together a militia. When Shay's Rebellion broke out among soldiers In Massachusetts, it was put down by the state of Massachusetts and a privately-funded local militia. The national government could barely maintain order, let alone regulate commerce or secure profitable trade with Europe. It was just a loose association of the former Thirteen Colonies, waiting to be made into a stronger national government.

In 1787, just after Shay's Rebellion, George Washington attended the making of the new national government, the Constitutional Convention.

In existing state governments, people of wealth and social standing naturally assumed their positions as members of the ruling class. And while the Revolution was fought by a million men both rich and poor, the Constitution stood in sharp contrast. The writing of the Constitution takes place behind closed doors in the Philadelphia Statehouse, between 55 delegates of the nation's leading aristocrats.

There is a specific current of anti-democratic thought between many of the Founders. Alexander Hamilton had mentioned before that no government could succeed "which did not unite the interest and credit of rich individuals with the state," and that "The people are turbulent and changing; they seldom judge or determine right." Hamilton, whose mission was to give business interests an interest in the government, also wanted Senators and Presidents to rule for life. The wealthy plantation owner Charles Pinckney proposed that no one should be president who is not worth at least 100,000 dollars, etc.

As a wealthy plantation owner himself, & the wealthiest president that doesn't fall within the 20 year rule on this subreddit, of course Washington wanted a strong national government. But it's his reluctance to assert himself and aggrandize his own power that gives him nicknames such as "The American Cincinnatus," after the Roman military leader who famously resigned and gave up his own position of power. Washington had already played in active role in two long wars and the making of a few governments. He did not have much political skill or a desire to wield power, only coming out of retirement to attend the Constitutional Convention. Now in his late 50's, he very much wanted to live out his days on his Mount Vernon estate, and live a quiet life managing his plantation. He was the same man who had to pull out his spectacles and put down the Newburgh Conspiracy, and at this stage, he also dealt with recurring malaria, episodes of dysentery and tuberculosis. By the time he was president, his health was in decline. Still, he was inaugurated to imbue the national government with his force of popularity and the acceptance of its strength, with one real tooth left in his head when he was sworn in.

With the upper classes already controlling the fate of the country, people had tried before to convince General Washington that he should rule as their monarch, and he voiced a harsh distaste for it. He was reluctant to wield power, but he also wanted to see a strong national government that would be capable of collecting taxes, with its own revenue and the power of its own militia—one that could protect property rights, maintain social order for Southern plantation owners, and prevent such rebellions as Shay's. A strong national government could also regulate commerce and foster a strong European trade for the Northern business class.

It is still remarkable that the Founders formed the nation's first stable republic, with democracy among the upper classes and the balance of power it maintained between the Northern business interests and Southern plantation owners. But elections took place only among men who owned property, unless the states got rid of property requirements for voting. The Constitution did no such thing, and many poor tenant farmers would not get the right to vote until 40 or 50 years later. The new national government, of course, relied upon the ratification of state legislatures to lend it legitimacy.