In ME/CFS, it was shown in the 1980s that ME/CFS patients have enterovirus infections in their tissues, but even today it has not been proven this virus causes ME/CFS. Science is hard, and answers take a long time coming.
Also, sometimes scientists have their own views, and they like to stick to their views. I tried to communicate with the researchers in China who are studying the Chinese HIV-like virus, and I suggested that they look at enterovirus as a possible candidate. But they were fixated on herpesviruses, because of the herpesvirus reactivations that the studies on the Chinese virus found. So I could not get them interested in enterovirus. But that's how things go: individual scientists have their own ideas and research preferences.