Why is the biomarker so elusive?
I think it's because almost no money has been allocated for ME/CFS research for the past 40 years. I saw a video the other day where someone was praising the NIH because ME/CFS funding had doubled within a few years - right - doubled from almost nothing, $8 million a year in 2016, to about $15 million, which is still almost nothing, compared to any other serious illness. This comes to about $7.50 per ME/CFS patient per year, based on a guestimate of a 2 million ME/CFS patient population in the U.S.
Funding for AIDS is in the thousands of dollars per patient per year and AIDS patients (except end-stage) have a much higher quality of life than ME/CFS patients.
MS funding is $126 million per year - this comes out to about $126 per MS patient per year, based on an estimated 1,000,000 MS patients in the U.S. So MS patients get about 17 times the funding ME/CFS patients do, and yet on the whole we are much more disabled. Plus there are at least twice as many of us.
I think the money (or lack thereof) speaks for itself in terms of finding a biomarker or anything meaningful.
I think the ridicule and scorn and disbelief heaped on ME/CFS has contributed greatly to the lack of funding. And almost no none wants to risk their research career on an illness which don't get no respect or money. Though I think Long Covid may be a wake-up call that hey, this invisible non-existent illness which can't touch me might be real after all, and those who hold the purse strings of federal funding might loosen them a bit, though we're still waiting.