oooooh! Philosophy!
I think there are many ways to interpret this but the end result is what counts.
Your art is an expression of an idea within a certain constraint form. Music in this particular case.
I speak for myself but maybe someone can concur:
An idea transforms into lyrics or music and becomes materialistic and measurable. You take it out of your head and actually see the outcome (hello to the judge your music objectively - thread).
The outcome might be misaligned with the way you wanted.
It might appear unworthy once examined from this different angle
Your-self might have changed between inception and production, so the idea does not appeal to you.
It can just generally suck too, but I don't see any objective way to distinguish suckiness from the above. (unless someone sings out of key, that thing ... don't get me started)
So the end result is proportionally smaller than the intent. That's how it is. It's hard to push your music to audiences as it is, imagine what would happen and how would you feel if every idea anyone had would end up fully made.
On a side note, this is what I heard about the greats:
Linking park had in interview in the early 200 - their 3rd album had over 90 songs and it got narrowed down to 10+.
Michael Jackson admitted on stage that each album compiled of about 150 songs in various stages of production.
Picasso did about 2 pieces a day. I think he regarded most of them as crap unworthy of anything
Image result for picasso how many paintings
Picasso was exceptionally prolific throughout his long lifetime. The total number of artworks he produced has been estimated at 50,000, comprising 1,885 paintings; 1,228 sculptures; 2,880 ceramics, roughly 12,000 drawings, many thousands of prints, and numerous tapestries and rugs.
To conclude - all we do is dig for gems. Good pieces are based on multiple attempts at crap and mediocrity and we are in a good company