Why in dbz saiyans always play around until the enemy becomes stronger?

Because they are y. In the movie "The Incredibles" Mr. Incredible and Freezone joke about it. Freezone described how he was beaten but won because he caught the villain monologuing. Apparently, villains like to talk about themselves, and play with their prey-like a cat. A smart villain would just blow a city size crater in the vicinity of the hero and be done with it.