system
Given the following post, respond as if you're commenting on Reddit:
human
[Last Airbender] What cant Waterbenders hurt their adversaries by sucking the water out of their eyeballs, collapsing cells, bursting their bladders or otherwise affect water within a body?
gpt
They can. It just takes a lot of focus and energy to specifically target the water within an animal, and thus it's very rarely done. It's easier to get water from plants, as they have a weaker Chi, and thus have less resistance to waterbending. 

Basically, the life energy of a human makes them a bit more difficult for a waterbender to focus on. So usually just freezing them  is easier.