Why do developers write instead of reuse?

I am frequently amazed at the amount of software that is being written instead of simply looking around and reusing what’s already available. In practice I have seen a lot of reasons for this:

• Our problems are unique: The misconception that “our problems are unique”. I really can’t recall how many times I have seen this but this is really occurring a lot.
• Not looking for similar solutions: Simply forgetting to look for similar solutions on the internet to see what’s available (if only as an inspiration on how to best solve the problem). This is often also a side effect of thinking that this is a unique problem.
• Underestimation of the problem: The misconception that it’s easy to write it yourself. In most cases, it is easy to come up with a first (half) working version that does approximately what you need. However, the work involved in making the same solution maintainable and with the correct feature set will make it much more expensive (the 80-20% rule).
• Limited scope: A developer specialized in platform X (e.g. X = java) will typically only look for solutions in that area, whereas looking broader will reveal more solutions.
• Coolness factor: It is cool to develop it yourself. Perhaps it involves an opportunity to do something cool with clustering or another chance to use one of your favorite frameworks. Perhaps you could use one of those cloud databases?
• Overestimation of oneself: The idea that we can do something better in a few weeks time than what the industry or open source community has come up with using man years of development.
• The desire for fame by writing reusable software: Paradoxically, the desire for reusable software can stimulate to roll your own. The problem is that writing reusable software (or calling it reusable) provides you with fame (even if it’s only in your local department). The reality is however that reuse can only exist through the willingness of people to use other people’s software. If there is one developer writing a reusable piece of software and 20 others using it, then clearly the willingness to use other’s software far outweighs writing it yourself.

I have seen these problems in companies of all sizes.