It's a matter of probability like everything in life.
A good parent can lead to good outcomes, say, 60% of the time. A bad parent, 20% of the time. The numbers are random, but that's the idea. In retrospect, the vast majority of parents I knew when I was a kid were "bad," not in the sense of abusive, but in the sense of having no idea. They didn't have any clue about their own lives, how many could they have about their children?
They didn't even want to inquire and then decide what was best for their children. Too many parents are given a pass because some sort of "sacred virtue" is assigned to parenting and any mistake that does not include beating their children is somehow forgiven or given a pass.
The "they did the best they could" is a cop-out: did they do the best they could given their laziness, poor impulse control, vision that doesn't go beyond the fence of their backyard?
For example, I was an excellent student, the best in the school, in terms of awards and recognition. Not Einstein, mind you, but someone they should have identified as very talented from a scholastic/intellectual standpoint. My parents never saw this in me, for some reason, because now, I would immediately identify as very talented someone who did as well as I did.
My parents never hit me or told me I was stupid or anything like that. They simply acknowledged that I was a good student and that was it. If they had said, this woman is talented, we need to support her, find her mentors, send her to the best universities, I have little doubt that I would have been very successful.
I was successful anyway, after traveling a winding and long road, but my path to professional and financial success could have been (these are my a priori odds) much easier and faster.
Now, you might say, if they had acted differently, maybe you wouldn't be here, etc., which is a very defeatist way of thinking. Their process was wrong and for no other reason than their "intellectual laziness".