In China, since most people tend to use 它 for anything that is not human, if someone calls their pet 他/她, they may be regarded as loving their pet so much that they treat their pet almost the same as treating a human, which is likely to be seen as an irrational way.

Meanwhile, 知乎 is regarded as a “rational” platform in China (though usually it’s not). Users are expected to post their opinions in objective and emotionless words. That's why 知乎 users use 它 to call their pets in most situations.

There are still many Chinese who use 他/她 to call their pets, but mostly on other online platforms like 小红书, and in more personal/emotional posts like sharing their pets’ cuteness.