Comment by grishka

> I don't know how we got here

I have a suspicion — it has to do with instant messaging clients. The idea being that you want to type short one-line messages and send them as quickly as possible in most cases, but in a rarer case when you do want a line break, that's Ctrl+Enter or Shift+Enter. Probably the first one where I personally encountered "enter is send" is ICQ, but I'm sure it's older than that, I would be surprised if no IRC clients did that.