I think they have to be trained to be friendly.
Our pond was here when we moved in. It already had koi, and those were very shy. That set died in the aftermath of a hurricane.
Then we slowly repopulated the pond, and the new fish were/are just as skittish.
We don’t exactly go to extreme lengths to raise our koi. We feed them and take care of the pond, but we don’t intentionally breed them. I think a dog or cat would be more work.
Honestly, when it comes to effort, I’d like to ask why do people expend the effort to keep pets? Our original koi came with the house and happened to fit into our lifestyle and we grew attached. For us, they made sense.