I would argue that we convey the same meaning during speech by changing pitch, stressing certain vowels and consonants, etc. Things that we can't do in writing without things like apostrophes.
This is a good point. The way I usually think of this is that most of the time we simply don't need as much redundancy as we have encoded in language. It only becomes important in instructions for complicated things, scientific papers, or when you are communicating over the top of loud noises or a bad phone connection. I don't think explains the forces that shape our language fully, but it does provide a starting point!