"I got used to living in an unstable situation and figured somehow it would work out"
That last bit is illusory thinking.
Also, I'd beg to differ on your suggestion that these people aren't choosing their life every single day. I'd suggest the opposite is true for most of us (leaving aside those who through incapacity have diminished options). Every single day you don't make an effort to look for work, don't volunteer for something, don't organise an activity, don't spend some time learning something, don't go somewhere to meet people, don't go on a date - you are making a choice. Living with your parents and (more importantly) living off your parents well into your fifties is not an accident. You can't accidentally do that. That is directly the result of daily choices and illusory thinking.
And while we can agreee that the term "parasite" is pejorative, I find it is deserved- at least from the perspective of the parent (who in this case is probably as much to blame for this situation).
My daughters are 20 and 18 and the expectation has long been set in our family that they will be moving out once they're in stable employment (and by that I mean any job that can pay their bills, not their dream job). When both kids are out of the house, we'll downsize and share some of the cashed out equity to help them get started on the property ladder. Then they're on their own. In our family we refer to this as "The Plan". They've been hearing about the Plan (and been onboard) since their early teens.