Disney International released the single internationally on June 23, 2003 (see 2003 in music). The CD single release included a remix of "Why Not?", a remix of "I Can't Wait" and the music video. Later in 2003, a different version of "Why Not?" was released on Duff's first album, Metamorphosis. According to Duff, the song is also included as a single for the album Metamorphosis.
"Why Not?" has three versions. The main version is the one originally recorded for the soundtrack of The Lizzie McGuire Movie. The second version has slightly different lyrics in the first verse and was released as a second single on her album, Metamorphosis. The third version is a remixed version of the second version, with a rock arrangement; this version was released on Most Wanted. The re-recorded version is included in the collection of Duff's greatest-hits album, Best of Hilary Duff.
Singer Michelle Branch said, in a 2003 interview with Rolling Stone Magazine, that she felt it was, at the time, the worst song she had heard recently: "One lyric stuck in my head: 'You always dress in yellow when you wanna dress in gold,'" she said. "I heard that and I was like, 'Oh, man, what is going on with music these days?' But, hey, whatever floats your boat." It was not known, as of the middle of October of 2013, whether Branch knew at the time that Duff had neither written nor composed "Why Not?" herself.
The video, which was shot in Downtown Los Angeles, first premiered on the Disney Channel in April 2003. The video became the first of Duff's to air on MTV after a viewer requested it on the popular show Total Request Live. The video debuted on the show's countdown at number six a few days later. The video shows Duff singing from a rooftop; this is inter-cut with scenes of street scenes of skipping, break dancing and crowds running, as well as waist shots of extras mouthing the words. Two distinct versions of the video exist; the one includes scenes from The Lizzie McGuire Movie while the other does not.
The single debuted on the Australian ARIA Singles Chart at number forty in week twenty-six of 2003, and it peaked at number fourteen in its twelfth week, remaining on the chart for sixteen weeks. The song also reached the top twenty in New Zealand and the Netherlands.
^Duff, Hilary. "LA". HilaryDuff.com. January 27, 2003. Retrieved August 26, 2013. "I finished recording my next single, called 'Why Not,' and it will be in the Lizzie movie. I'm working with my vocal coach every day and we're getting ready to record my album."