Why did George Gershwin usually compose melodies before Ira Gershwin wrote lyrics?

The songwriting process of the Gershwin brothers was unique because George Gershwin’s melodies typically preceded the lyrics, reversing the standard practice of most contemporary composing teams.1

This approach was driven by the nature of George’s musical imagination, which was so facile that he could compose quality songs within minutes of improvisation. Additionally, he frequently drew from extensive notebooks of song sketches he had accumulated over time, often selecting and embellishing melodies he had previously labeled as "g.t." (good tune).1

Once George provided the melody, Ira Gershwin would spend a week or more meticulously fitting words to the music. Ira’s dedication to polishing each line—often working within highly confining rhythms and accents—earned him the nickname "The Jeweller" among his peers. When asked by interviewers which element came first, Ira famously quipped that the only thing that truly came first was "the contract".1