Hi
Great topic and well picked up by so many. You've had most of the answers I would had given and a few that seemed fairly random. I could add a couple more things:
Someone above wrote about the standard practise of being given a set of chords and the task of writing something within them for college - Great exampl! Plus, it defines just how difficult it can be - if not impossible - to copyright a chord progression. This is where the melody becomes the signature that defines the music.
There are many reasons given why particular song structures are often repeated and indeed, some of these progressions; e.g. the ii,v,i (2,5,1) in jazz and blues have become hackneyed old cliches. Even in classical music, there are harmonic structures used as vehicles for specific musical actions. there really is nothing that original.
Years ago, I decided only to write music which didn't follow any harmonic patterns which I considered corny or overused. I found myself bending my fingers into contortions trying to, er ... 'develop' a chord shapes. After years of listening and learning I realised that there's more goes on in a song's arrangement than in the writing. The chords of a piece of music might be lifted straight out of a million songs, but the melody, tempo, time signature, arrangement, rhythm, musical stress and, of course, performance are what define it.
Incidentally, there is the chord progression, which can be overlaid for:
Stairway to Heaven
Feelings (Albert Hammond)
My Funny Valentine
Various Beatles tunes
The Sweeny Theme Tune (UK viewers)
given time, I bet could find a whole lot more