Obviously people get stronger when they fight, fighting is the best form of training. However; Saiyans don't just get stronger from fighting, they train, Gohan first achieved SSJ2 while training. Saiyans train much more than Naruto characters, they train 7 days in a week. They derive a lot of their skill from training, while Naruto characters often learn in the heat of battle.
Kakashi doesn't just magically get stronger, or wait for battle, he trains. It is obviously not shown, 3 years of training were skipped. Training doesn't exactly make for a good manga, so Kakashi is never shown training. Obviously Naruto is the exception, as Naruto is the main character.