I tend to agree with you. Rather than telling you why they can't (because they obviously can) I'll suggest how they might do it. Whether it is viable to do so vs what else they could be doing is a question I can't answer.
First thing people raised is "what is optimum". Great point. However, it is entirely possible to set a more specific target. For example, many individuals mentioned ASW. A reasonable testable value would be % time in ASW. I would suggest you could give settings for minimal ASW (0 - 25%?), some ASW (25 - 75%?), and frequent ASW (75 - 100%). The actual values are irrelevant but the fact that they can be chosen, tested, and published are what is important.
Second, there is a great amount of variance in PC's from drivers, config, etc. However, this hasn't stopped anyone from publishing minimum requirements, recommended requirements, and from thousands of sites running tests on hardware and games and publishing them to the internet over the years. They would simply need to state the system components they are using to give people an idea of what their system is like in relation to it. (Swapping out graphics cards obviously.)
Third, could you just pick a setting and tweak it from there? Sure you could but you are asking if they could do it as they have already done testing. So this statement is irrelevant to what you asked.
Finally, while they certainly COULD test graphics cards and tell us their recommended settings or give us some parameters, do they have the time and money to do it? I can't answer that one. I know you have stated that they have already done testing. That makes is seem like publishing the information would be free. However, we don't know if they were testing and recording the type of information that would be relevant to this discussion. It's quite possible that they would have to a completely different set of tests all over again to get what we need.
It might be better to let them keep coding and instead pick a setting and tweak it ourselves. Or perhaps we as a community could collect and publish the results on our systems to help each other out which I have seen in this thread.
Lots of possibilities here and I think your question is a reasonable one.