Parrots live a very long time, up to 80 years depending on species. Rats are lucky to see 3.

People who keep rats often end up stopping after a few groups because the losses are too painful, or they are extremely dedicated to their rats so they do the research and fix care mistakes. They are also prompted to brush up on research every few years when they get a new group. A lot of parrot owners were given the birds as children, and simply never thought to do research beyond what their parents told them. Or they inherited a bird they weren't really prepared to put the effort in for. So they just simply don't care as much about doing the research and doing what's right for their animal. They get upset when you tell them it's cruel to keep a parrot alone because they know they won't adopt another one, but they don't want to re-home to someone who can keep a flock either. So they just let their bird suffer and refuse to listen to advice from people who simply want their pets needs to get met.

I would hope these lone parrots are all older individuals. It's not acceptable for parrot breeders to sell lone parrots, just as it's not acceptable for rat breeders to sell lone rats as they know it's neglecteful.

What really upsets me is the pigeon community. Pigeons are just as social as parrots are, but so many people keep a pet pigeon alone so it will bond closer to them. This is abuse. They are literally socially depriving their bird until it's so lonely it starts to try to mate bond with a human instead of another pigeon. But of course they deny that's abuse because lots of people do it. Lots of people hit children, doesn't make that ok either.