or JSON or S-EXPR; or structures in memory with pointers between nodes (multiple ways to do that, also very specific to the current programming language) and you can store that with some serialization mechanism; or records in SQL table with links as foreign keys (nasty); or object oriented databases (overkill)
It is hard to do them all, or to come up with one solution that pleases everybody, therefore everybody is doing his own tree. Maybe that's not the most general solution, but in reality you really need this one particular choice of tree representation to do your job.
