Mr Popo layin down the law

also I think the idea behind not fighting at 100% from the start is that they can learn about their opponents abilities while conserving some stamina. They also want to avoid showing the enemy their full potential if they don't have to. They also probably don't want to unintentionally annihilate every weak opponent.

of course the real reason is to drag the fight out for more air time and flashy transformations.