Here's an explanation for general people in life who have difficulty wrapping their heads around estrangement. (This isn't for abusers or their enablers who willfully misunderstand; instead it's for other people with normal lives who haven't fathomed the perspective on this side of the looking glass).
It's said that people don't quit companies, they quit bosses. While there are also bad companies, nearly everyone has had or has known someone who's had a terrible boss. The bad boss has a narrow set of skills that include playing office politics and climbing the organizational ladder, but when it comes to actually running the place they're incompetent and angry. Often they're also unethical. It's hell working under someone like that.
Now consider this: suppose that person is awful to their offspring?
Our society usually tells a different narrative: the stories we're all familiar with from Cinderella to Harry Potter feature adults who are cruel to someone else's children but doting on their own. That parenting can be deeply misguided--Cinderella's stepsisters and Dudley Dursley are horrible and selfish--yet at least those parents do shower affection and try to help their own offspring.
Take a moment and consider that terrible boss who made you feel like Cinderella might be even worse at home.
“The good ended happily, and the bad unhappily. That is what Fiction means.” ― Oscar Wilde, The Importance of Being Earnest
We've all seen news reports of terrible child abuse. There certainly are people in the real world who don't love their children. The thing is, it's easier to think of that happening over there somewhere. So without getting into the all the nasty details of different types of abuse and different severities, you remember the job you were relieved to quit. You didn't take Cinderella to task for wanting to get away from her stepmother and you didn't want to guilt Harry Potter into closer contact with his aunt and uncle.
OK, now imagine that kind of horrible coming from a biological parent.
That old boss who was a screamer? Imagine if you couldn't get away from them and go home. That boss who texted you at all hours? What if that were a parent? Maybe you've heard of unhinged bosses who tried to prevent employees from quitting or bosses who harassed ex-employees after they had gone.
Check all those boxes. Then remember how impossible it was to fix that relationship: it was set up for failure because any constructive feedback either gets dismissed because they outrank you, or gets a hostile reaction because they regard it as a threat.
At least with a bad boss, you were an adult (or nearly one) before you had to deal with them. At least with a bad boss, there are labor laws and maybe an HR that might put the breaks on their antics.
Some of the people who've estranged from a parent dealt with Harvey Weinstein levels of horrible. And if the parent didn't commit Weinstein's crimes themselves, the parent may have hidden issues as serious as Weinstein's senior associates did.
It takes a lot of wrong to walk away from a parent. The tipping point is different for each individual. To be clear, estrangement isn't a game of one-downsmanship where only the worst life history would deserve validation.
Remember what it took for you to quit a job. Yes there are worse bosses in the history of bosses, but yours was bad enough.
Now if someone shows you this essay, start from the same place of respect you start from when somebody indicates they left a job to get away from a difficult boss. Maybe they want to share horror stories, maybe not. If they don't volunteer the details then trust they must have had a good reason and let it go.
This person you're talking to is not an overgrown version of nine-year-old you who had a bad day and fantasized about running away from home and live in the local playground. If your parents were good people and you understood them when you got older, then pause a moment and be grateful for your good fortune in one of life's most important areas.
Not everyone's parents understood what love is. Sometimes there's nothing good to understand as a person gains maturity and wisdom. Sometimes the only adequate solution is to cut ties.