Reliability and lack of information. The problem with a "format" such as timestamps is that there is no defined parameters, especially cross language. I cannot rely that sending a timestamp in seconds will be read in seconds the other side (for example JS does not use seconds, it uses ms), there is no way to tell what one is over the other, either. I cannot look at a ms timestamp and say "This is in ms" over a second-based timestamp. It also lacks information, such as tz, and is of a fixed specificity. What happens when I want higher resolution time due to customer feedback? Go back and change the whole system to use MS over S.

ISO8601 defines its parameters within the body. I know I am parsing seconds from an ISO stamp because it is appended with an 'S'. It can support variable specificity, so I don't have to be second-accurate if it isn't needed.