Josh Groban is a cute duet partner, before I googled his photo, I thought it was Jonathan Groff, glad I was mistaken. This makes me a little bit more excited for the duet, although I'm sad that it is some song from a musical, not an original song.
How comes that all people who collab with Kelly collab with her when they are about 10 years past their peak?

The only ones who collabed with her during or before their peak were Jason and John.