Because that’s whats written in the score.

As everyone has said, it’s wrriten as such. But a reason was the first Glinda (Kristin Chenowith, who the role was basically created for) was a classically trained opera singer as well as a broadway performer. She offered the idea to do an aria in the song, they hit it off and into the sheet it went. And since then it continues to pay off 🩷

Bc she was lying