Yet everyone insists on solving them afresh.

I'm exceptionally guilty of this, but I feel I have good reasoning for it.

The main reason I approach problems already deemed "solved" is to learn. Now you may say that I could just as easily learn from reading how others solved them but I've found narrows one's approach to problem-solving. One becomes set on a particular method of problem solving and unable to approach problems from a different perspective. As an example, consider when most major scientific achievements occur in a scientists career - it is almost always early on when they're not so set about their ways.

Also too often I notice that after such research, one does not truly understand the subject matter. After studying an algorithm, rarely can one write the algorithm on their own, test it effectively, and improve it for specific circumstances. After creating one from scratch, this is almost always easy. As Richard Feynman once say "What I cannot create, I do not understand."

The second reason I prefer to start fresh is that the software industry (including open source software) today is a mess. The amount of unecessary complexity that exists today is a major hinderance to anyone trying to learn a new task. Designs evolve in an unconsistant manner over time and result in a lot of unecessary subtleties that cause you to spend more time looking at the tool than the task at hand. I'd like to think that if I can design something from scratch, I can provide the consistancy required to maximize learning efficiency.

Then again, maybe I'm just suffering from a severe case of not-made-here syndrome :)