They are no longer in the wild which is basically the reason they harm themselves. People fail spectacularly at providing for the needs of parrots and this is not always our fault. Naturally we can not give them the exact environment they encounter in the wild and some of the issues that cause mutilating and plucking are because of this fact. We can not recreate their natural diet right now, we can not recreate a flock that is 50 or more strong, we can not surround or be near them 24/7, communication between humans and parrots can be faulty. Parrots in sanctuaries with flights can do very well because they are much closer to being a natural setting (lots of birds) than our homes.
Plucked feathers can start from anything from diet, health issues, environment, that weird looking statue won't stop looking at me.... A lot of the time plucking may start because of health issues but become a behavior issue after the health issue goes away.
Plucked feathers can and will grow back. It is when plucking becomes chronic that they damage the follicles, no longer allowing feathers to regrow.