I can kinda understand why ChatGPT and other chat bots do it. It's a chat interface. Most people chat with single line prompts.

Next door and social media apps, to answer your question, I'm sure a PM somewhere was able to prove that engagement increased if we let people share their thoughts immediately, and the PM got a tidy bonus because of this.

I would be OK if they put a checkbox next to the text input that let me choose whether enter sends or line breaks. I would be OK even if that lived in session storage, to remove the friction of a new Db column. Just give us the option!