I agree that "if both sides are emotionally healthy and respectful, there would be no need to be here" but sadly when it comes to estrangement that is rarely, if ever the case MamaBear, hence the estrangement.
I also agree that it is often a last resort but it very much depends on why it's a last resort. For example the AC of an abusive parent estranges as a last resort to be free of their abuser.
An AC who marries or establishes a relationship with someone who is emotionally abusive, controlling and manipulative may see estranging their parents as a last resort to maintain their adult relationship, especially when they have children of their own, and want to keep their family unit in tact for the sake of their children.
It can also be the case that because of the subtle control and manipulation they've been subjected too over a period of time, they are blind to the extent they've been manipulated 'believing' that the estrangement of their parents is justifiable.
Although it is all too often disregarded, especially here on GN I've found, the importance of a third party's influence should never be underestimated.
There are numerous examples of how a parent's relationship with their AC is undermined by that AC's partner. There are currently examples being discussed here on other threads where this is the case.
If it's possible for the mutual love between an AC and his/her parents to be undermined by a third party which it is, it is small by comparison to accept that the mutual respect they once shared can be undermined too.