system
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human
[Avatar: The Last Airbender] Couldn't earth benders just bend small rocks and hurl them at the speed of a bullet

It seems much more effective then slowly throwing big rocks at your enemy, or cracking the ground beneath them.
gpt
For the same reason most airbenders don’t just bend the air out of people’s lungs, most firebenders don’t just fire jet engine streams out of their hands, and most waterbenders don’t just strangle people with vines by bending the water in them. 

Because it’s not actually that easy to do. Bending works off of specific styles and forms, and — like all martial arts — it takes years of practice to achieve competence. More specific and esoteric styles exist, but they’re not common and likely take even more years of specific study.