2 problems when you have a system of self declaration of fatigue:
1) If you are fatigued, your judgment is impaired, so are you making the right call? Kinda like having the drunk decide if he's had too much to drink.
2) I might be fine prior to pushback, but how am I going to feel in 6-8-10 hours when it counts?
And we've got to decouple pay from safety related cancellations. A few years ago Valujet flew a 9 into a thunderstorm and totaled it, lucky to get it back on the ground. Radar hadn't worked for weeks, but if you turned the jet down, they docked your pay.