Why did the artist paint George Washington standing in front of the delegates?

They don't. At least one famous painting and several statues show him on horseback. And you can't tell if he's standing in the well-known head-and-shoulders portraits, but most likely he posed seated.

However, a standing pose is appropriate to a leader. Being shown standing tall and larger than life, just lilke the Eurpoean kings, inspires the people to look up to him. Many people might see the portraits who were never going to see him any other way; the camera had not yet been invented in Washington's time, never mind television or film.