Well, that's simply not true, and is a classic example of why people avoid joining this forum, including myself. There are many biopsychosocial studies with objective outcomes, e.g. grey matter, cortisol awakening response, autonomic response. The problem is that these aren't really biomarkers for ME/CFS. There is a lot of talk on s4me about using walking distance as an objective marker, but the problem is that walking distance isn't a biomarker of the illness either. Mild patients (who are the ones in these studies) are not likely to show any difference in walking distance when they recover.
The reality is that at the moment, self-reported outcomes are the only measure of the illness. That doesn't mean that self-reported outcomes are useless though. There are things that can be done to reduce bias, such as having an attention control, and having long-term follow-up. It may also be possible to have a trial on bedbound patients, as they are likely to show increased activity.