In some cases the possum was taken in as part of a rehabilitation program and could not be rewilded so they are kept as animal ambassadors to show people how chill of an animal they are.

...and in other cases someone just snatched up a possum from the woods and named it Gretchen.

They aren't domesticated but they're so chill it almost doesn't matter. I don't recommend keeping one as a pet though; they require specific dietary needs and don't live long.