Because conversation doesn't (and shouldn't) work like that. There's plenty of asides, or casual mentions of things that are only slightly relevant or add to the point without being all that important.

Say we're talking about genetic engineering and I say something like 'This reminds me of that one movie where the one like normal guy, I think it was Tom Cruise, is trying to go into space'. It doesn't matter that I'm talking about Gattaca, or that it was Ethan Hawke and not Tom Cruise (a name just pulled out of the hat) and maybe even messing up plot details, and stopping to look it up just completely ruins the flow and break conversation.

I had this happen in person last week when someone mentioned Irish had a word for a three month period, but nobody could remember it. Several people went to their phone dictionaries immediately and it just stopped the conversation (and none were able to find it, either).

So there's lots of times we bring stuff up just as an aside or to add a little more flavour without it being important in conversation. I'd say that's hoe conversation should work not just 'Here's my point and all the facts that back it up.' There's no flow there, it becomes more like a debate or presentation, not a conversation. And if people are curious or it really was important, you can always look it up and send it later, but you'll often find nobody is that curious and it isn't that important.