Why is column default value not portable through different RDBMS?
An ORM that is intended to be truly storage-layer agnostic will generally assume the absolute minimum from the data layer (tables, rows, columns of standard types, indexes, PKs, FKs, and that is about it) and will implement everything else like defaults and more complex constraints themselves.

While every relational database I could mention (mssql, oracle, prostgres, sqlite, mysql, ...) supports default values as defined in SQL-92, things vary a lot more once you consider using document oriented DBs and other less traditional arrangements for your storage layer (they might not support them and if they do the operating semantics may not be quite the same).