You forget that the goal is to get ratings. Special appearances and performances by the likes of Vonzell, Maddie Poppe, Just Sam, Kellie Pickler, Blake Lewis and Carly Smithson aren't going to get casual people to watch the show AT ALL.
On the contrary, people may tune in if they hear that Ben Platt, Bebe Rexha, or Flo Rida are performing since they are well known/popular/relevant.