You can check for intoxication and get a number. Tiredness and fatigue are akin to hypoxia as they are difficult to self-diagnose, let alone check for. How do you define “unfit to operate”? What level of tiredness is that? How do you measure it? Any sort of flying produces some level of exhaustion, minor or otherwise. Does this make night flights impossible?
Commercial pressure and regulatory capture have made a farce of FTLs in EASA land. I wouldn’t sit next to someone whose blood alcohol level was impairing their performance but I do it all the time with people who are tired. It’s the nature of the job...