Keep in mind: a unit's mobilisation performance (turnout rate and swiftness of reporting) is also a career KPI of the commanding officer.
If not for avoiding punishment, do you think anyone would bother to turn up?
If SAF/Mindef do not believe me, I challenge them to organize an experimental 'voluntary mobilisation'. Turning up is purely voluntary, and you won't be punished for being absent. Try it, show the numbers and let's see how 'operationally ready' the SAF really is.