[bmitc]

I would argue that a huge or maybe even the primary reason why maintenance is so hard, though, is because a lot of software was originally not written "correctly", that is without maintenance and longevity in mind.

The hardest part of maintenance, in my experience, is that programs were developed under needless or incorrect constraints and then expected to be magically maintained. There is a huge downstream effect of decisions made early on in a software system's life.

The story of "just get it working" that evolves into "now that's it's working, don't change it but add these new features" repeats itself over, and over, and over. It's not surprising why maintenance in systems developed like that is hard.