Why did George Washington warn againist political parties?

Because he was afraid it would divide the country.
He wisely knew that it could divide our country.

George Washington's Farewell Address

"They serve to organize faction, to give it an artificial and extraordinary force; to put, in the place of the delegated will of the nation, the will of a party, often a small but artful and enterprising minority of the community; and, according to the alternate triumphs of different parties, to make the public administration the mirror of the ill-concerted and incongruous projects of faction, rather than the organ of consistent and wholesome plans digested by common counsels, and modified by mutual interests."

This is what we have today politicians that represent the rich and powerful not the majority of Americans... not national unity...