Why can't you just always use hand-written JDBC instead of an ORM?

You can, for genuinely trivial single-table CRUD — but for any real object graph (parent-child relationships, inheritance, deep navigation), hand-written JDBC means manually writing and maintaining the INSERT/SELECT/JOIN mapping code for every entity, and manually reconstructing Java objects from flat rows every time — repetitive, error-prone, and expensive to keep in sync as the schema evolves. ORM automates exactly this translation.