I think this one is easy to explain: it's a clash between chat idioms and forum idioms. Chats since IRC have had Enter=Send because messages were supposed to be single line, while web forums have had multiline text editors from the start so Ctrl+Enter=Send made the most sense. As the two merged, confusion and conflict was inevitable.
We can easily fix this by just letting everyone choose, but no one wants to make configurable UX.
> while web forums have had multiline text editors from the start so Ctrl+Enter=Send made the most sense
This is normal/standard behavior for multiline inputs such as the textarea, while input type=text/search/email will trigger a submit.
https://html.spec.whatwg.org/multipage/form-control-infrastr...
So basically a chat input based on web technology is saying:
"I'm a single line input, but will allow multiline editing if you ask nicely"