How do the singers/band thought up melody of their songs?

Thousands of songs use the same chord progressions, or patterns. I know because I play them. A really popular one is Em G C D. Pop songs today usually pick a few chords and repeat them over and over again. I know when I write a song I find some chords in a key I like, and I pretend that i'm listening to the song I'm writing, on the radio. The way you say the words can also change where you want the note to be. Because so many people like these patterns and melodies, artist don't need to change them up too much to become a hit. Then there are people that get the rights to old songs, and make them new.