The problem is that 19E thinks it's better off creating new stars on stupid spinoffs that they have total control of rather than hiring former stars which they can't control. Should George Huff become a lot more popular, for example, they wouldn't have him under contract to financially abuse like they do the new contestants.
And an "all-star" AI would only work if Simon, Paula, Randy and Seacrest agreed to be a part of it. Otherwise, it would be amateur-hour all over again. For some of those past contestants it would be a huge step backward.