It's an incentive question: Researchers are often evaluated based on the papers they produce (impact and numbers)--not based on how maintainable our code base is. If I can keep a cobbled together code base going and still produce code, then it's in my best interest to do science rather than write recyclable code.

The other problem that happens is that some people don't release their source code (they may release binaries) because either they're embarrassed by it, or because they want to maintain competitive advantage....