Writing your own data structures allows you to write it to suit the specific needs/constraints of your problem/domain. Typically that means you get something th

Writing your own data structures allows you to write it to suit the specific needs/constraints of your problem/domain. Typically that means you get something that is significantly simpler, faster and with a different feature set. Libraries are typically optimized for generic use, so are often more complex, slow, and difficult to extend with new features (both coding wise or the features might not be suitable / have trade offs for other people).