Because it is avoidable, but if it was not obvious that it can be avoided it was OUR mistake we will change it in future update!
In the meantime can you please specify how you do that?
I've tried killing all enemies on the spaceship, avoiding them, having the MC devoid of any lust, dpsing the final guy down, avoiding him and letting the timer run out, nothing worked. each and every time I got defeated even if my characters HP were full, and with the six months later transition lo and behold I get the unavoidable(at least as how I see it) cutscene.