Because total surveillance has never been goal or precedent.
The legal profession has always understood that people act differently when they speak and when they write.
It is unrealistic to hold people accountable to every mis-spoken word, mistake or dumb thing they have said in spur of the moment. If I ask someone 'how long will X take' and they must answer on the spot, I do not expect an accurate answer.
When you are writing, you have a chance to think through what you are saying. You should also be aware that these words could be read in 1 hour or in 1 day, and they still need to be understood correctly. You could be help accountable for the promises, whether legally, or informally by friends and family.
That's why the law recognizes the superiority of written versus verbal agreements.