Because the best endings are the ones with the hardest requirements to unlock. Not the happiest endings, mind you, just the most interesting, well written ones.
And Drakengard 3 in particular taught me that even horrible people who don't think twice about murdering can still do good deeds with good intentions and for noble-ish reasons. Zero did want to save the world and stop the Flower, even at the cost of her life, and she had very good reasons to hate the world.
Nier taught me even very good people can be horribly selfish and fuck everything up for everyone in their quest to get what they want. Nier essentially doomed humanity when he saved Yonah, but hey, she's free now so whatever.
Drakengard 1, by the way, taught me that horrible people doing horrible things for really bad reasons can still save the world, even if they don't really care about it.
There's more than one lesson in those games. But for real now, the reason I chose them was because I had just listened to The Last Song earlier and they were the first games with ridiculously hard/annoying requirements to unlock the best ending.
Any VN where the True Ending is locked behind multiple routes or game where you must defeat an optional superboss to see the Good ending would work to illustrate my point.
The best endings, not the happiest, not necessarily the True Endings, just the most satisfying, are always the hardest ones to reach.