Because it’s more effort to do and feels more artificial than using emojis. I’m not trying to craft literary masterpieces, I’m trying to have a conversation. With emojis, you get way closer to the feeling of a real conversation, where just the words are not the only signal that’s transmitted. Iirc there’s already some research into it, and it seems like people generally have started parsing emojis in text messages very similar to how they would parse tone of voice and facial expressions in a face to face conversation. I think it’s a great addition to written language, and you’re missing out if you’re not making use of it.

Edit to clarify: this applies to emoticons and emoji equally, even though I’m only mentioning emoji