Sometimes it's really just pilots being incompetent or drunk - cf. Aeroflot Flight 821 and Aeroflot Flight 593 as good examples (not exclusively a Russian/Aeroflot thing, but they have particularly egregious examples.)
And how is it that incompetent or drunk people are allowed to fly airplanes? That is still only a proximal cause of the failure; there are systemic issues if this is something that happens with any kind of frequency at all.
The system has humans and computers (the airline is a cyborg!) so human incompetence is definitely up there as a failure mode.
The pilot should not be drunk.
The pilot’s boss should not have let him onto the airplane while drunk.
That guy’s boss should have ensured that they breathalyze every pilot. And so on up the chain to the owners of the airline, who should appoint a CEO who can get the job done.
Breathalyze every pilot? Really? That sounds like a depressing, degrading work environment, which, when combined with the taboo against seeking mental health treatment as a pilot, might contribute to an increase in on-the-job suicide attempts.
In aviation, every knob you turn has at least two effects.