That's because the sensation of listening to an album gets intensified when you have the finished product in your hands. Even if it turns out below average you can still appreciate the work put into it if you hold the cover art and flip through the book.
Another example of this phenomenon is when people claim an album is absolute horseshit when in fact it's just worse when compared to the rest of said band's discography. In reality that album is pretty good when compared to other music and a ton of work was put into it.
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