My hypothesis: Lemmy has an older userbase, and in general older people feel less of a need to express their emotions. They're busier discussing the topic than highlighting their attitude towards it.
Perhaps cultural reinforcement plays a role, too. As emoticons and emojis are less used, they feel more out of place, so people who'd use them elsewhere avoid them here.
I have stopped contacting family members because the constant emoji spam kills all desire to have a conversation with them. Feels like empty meaningless chatter.
I only use emoticons to clarify an emotional message. On forums, I'm more interested in sharing and discussing ideas and opinions. Emoticon spam makes me sick with worthless cringe.