Yes, I agree that it's probably a speech thing. I had it dinned into me by my father, hence my pedantry on this one . Funnily enough I always say 'did you not?' in response to someone's view but 'didn't you?' at the beginning of the sentence if I'm instigating the question. Pedants' corner is certainly making me think!
I think I quite often say things that I wouldn't write. At least, if I did write them, I'd correct them, but speech moves at a faster pace. Also, things like "there's some more over there" trip of the tongue faster than "there are...." Maybe that's the main reason for many of these mistakes — it's just easier to say it wrong. At one time things like don't and wasn't were frowned upon. Where I grew up people tended to say "did you not?" rather than "didn't you?"
Yes, many of us get our plurals wrong, resulting in has instead of have. We treat singular group words as if they were plural, because the meaning is plural, in a way.
I often hear newsreaders and others saying the Government 'have' done this or that. Am I right in thinking it should be 'has'? Similarly with the word 'none' I've heard '...none have' rather than 'none has' but surely 'none' = not one and is therefore singular? I've heard these usages now for so long I'm beginning to think I'm either wrong or old fashioned