The problem is, you're dealing with only one side of the equation, our side. Frankly, there are countries—for reasons good and bad—that will not and do not as a common practice reduce these kinds of arrangements to writing.
Some very civilized countries interpret agreements in writing as treaties and require them to be brought before their legislatures. It would be difficult to do this for a whole range of things. Some countries have this rule, and make it a practice not to do this. Some follow this practice for other reasons.
We reduce what we do to writing so that the people reviewing it know exactly what the terms are. We separate our arrangements into three kinds: security screening exchange, intelligence exchange, and technical exchange. We reduce it to an understandable form, two ministers of the crown approve it, and the provision of intelligence and information is reviewed under these arrangements.
To insist that other countries cooperate with us only when the terms are reduced to writing seems a bit of a “bridge too far”—though I suppose we could say we will not do business with people who won't sign things.