ChatGPT does it.
Claude does it.
Nextdoor does it.
And none of those give you the courtesy of being able to turn it off.
Slack does it, but if you dig through the settings you may find the way to switch it.
How on earth did so many "designers" fixate on this idea that we must want to share our thought immediately instead of allowing a calmer interaction?
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!