"He was good at retreating because he had so much practice and no other options, since when do we praise generals for "retreating" better than anyone else?"
When?
How about when that individual continually manages to elude the grasp of much larger, much better equipped, much better trained, and much better led armies and still wins the BIG prize, Tele?
Keeping an orderly retreat, and keeping your army INTACT during such a retreat, is not only tough, it's virtually impossible.
There were a number of reasons that you're overlooking for the need to train, and when fighting, fight in a European style at that time.
First and foremost is that training in unit tactics instills discipline, which is critical no matter what the situation -- attack or retreat.
Second is how else would they have trained? The image of the American rifleman popping up from behind rocks and the like is a quaint one, but it wasn't, and couldn't, be the foundation for an Army.
In order for the United States to be recognized and gain the military assistance of foreign powers, it had to largely adopt the ways and means of those military powers. In the 18th century that mean using muskets instead of rifles, and using groups of men trained to march close to the enemy, hold their ground, and fire in vollies.
Even Wellington and Napoleon didn't break out of that mold, so do that mean that they weren't great military leaders?
Third, Washington was in command of the American army for around 8 years. In that time, units under his direct command won 3 battles -- Trenton (relatively minor, but one hell of a morale booster, which was more important), Princeton, which could be considered a major land battle in Revolutionary terms, and most importantly, Yorktown.
Princeton had a much greater effect, though. It caused the British to rethink their military strategy in the Northern colonies, and it led directly to the American victories at the Battles of Bennington and Saratoga which, if you remember your history, were the two victories that proved to the French, Spanish, and Dutch that the United States did have a logical chance at winning the war.
Finally, though, it was Washington's entrapment of Corwallis' army at Yorktown, his hammer against the anvil of the French fleet, as it were, that finally made, after all those years of retreating, the Treaty of Paris possible.
You're right, Washington wasn't a great field General, but he also never led anything even remotely resembling a quality field army. He was a political choice, chosen because he was a Virginia, but more importantly because he had the respect of his peers in Congress and the respect of the men he led.
Winning all of the battles sometimes just isn't important when, in the end, you still win the war.
Washington understood that. He understood that no matter what, he had to maintain a viable army in the field, and force the British to expend time, money, and effort to come after him.
I'm trying to find the refererences, but I think I once read something written by Ho Chi Mihn on this subject, which directly references George Washington's ability to lose the battles and win the war.
Looks like Ho Chi Mihn also understood that, because there's a single Vietnam today, not two.