Why do programmers write code in files, look it over, decide it's crap and rewrite it all within the same file instead of making a new file and removing the ori

Why do programmers write code in files, look it over, decide it's crap and rewrite it all within the same file instead of making a new file and removing the original? (Some don't even commit the old one for archiving, possibly even encrypted!) (Yeah yeah, not comparable. ;P ) I always liked Goya's work. I wonder if there's anything beneath "Saturn Devouring His Son"...