Because she cannot be bothered with singing. She's lazy with it.
I don't think she looks like she's heartbroken. She looks like she is still a 6-year-old girl lipsynching in the bathroom. Oh well! I guess we'll never understand why she does it.
How amazing would it be for her just to sing ONE SONG!
Calm down, know that the dance is geting better if we compare the FF tour the next step for Britney is do live at least Everytime, Lucky & Perfume Maybe one of this days she would serve live vocals