system
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human
[DBZ] Why doesn't anyone ever start a battle with their most powerful ki-based attack?

Every battle I've seen, every fighter (with the only exception maybe being Kid Buu) starts the battle off with a series of punches and kicks against their opponent who they clearly know won't be down for the count. This makes sense for villains who like to toy with their opponents, but for Goku & Co, they're DEFENDING the earth against people who want to destroy it and instead of just busting out the big guns early, every fight they have seems to be a war of attrition. 

tldr; Why doesn't Goku start off every battle against an villain by powering up to full strength and launching his most powerful Kamehameha?
gpt
There are a couple big things at play here:

1. Even when defending the Earth, Goku usually doesn't start the fight seriously. It's usually implied that Goku spends at least a portion of the battle holding back and testing his opponent to get an idea of their capabilities before seriously attacking. Other fighters tend to do something similar. This has both personal enjoyment reasons (most of the fighters enjoy actually fighting, particularly Goku who is a fighter first, and a hero second) and serious reasons. At the start of the battle each fighter is trying to determine just how powerful and what capabilities their opponent has. Both fighters usually begin the battle in a suppressed power state masking their full capacity. Without knowing what their opponent can do they might waste power, particularly relevant versus weaker foes who are being fought prior to a major enemy, but also versus a major enemy who might be capable of dodging an opening finisher.

2. Opening with a game ending Ki blast is risky. Powerful attacks often have a period of charging which telegraphs the technique, and this is further exacerbated by the fact that even attacks with little to no chargeup time can be avoided. Further, full powered Ki blasts, unlike punches and kicks, seem to use up considerable amounts of endurance. It's usually after using one of these finishing techniques that characters tend to state that they've lost a substantial amount of energy. Not going all in before you've put your opponent in a position where they have to take/oppose the attack insures you don't spend your energy pointlessly. Also if you miss, if you're a good guy at least, you have to make sure your ultimate attack isn't going to hit the planet.