The answer is PL/SQL does not support dynamic polymorphism. it only supports static polymorphism because
All PL/SQL generats a "DIANA" -> Descriptive Intermediate Attributed Notation for Ada , a tree-structured intermediate language. DIANA is used internally by compilers.
At compile time, PL/SQL source code is translated into system code and generates corresponding DIANA. Now think if there were a DDL statement like create table statement which at the compile time does not exists it will be created after running the program. how would your PL/SQL engine generate a DIANA then ????
The DIANA is plays an important role in PL/SQL to check/validate that the sub program. this is required because as we know that a sub-program can use database objects such as Tables,Views,Synonyms or other stored procs. it could be possible that the the objects may have changed/removed/droped when next time you run the program. For ex : some one might have droped the table, the stored proc or function singnature may have changed.
Thats why generally PL/SQL is used to manipulate the data within database structure, but not to manipulate those structures.
but there are ways to manipulate using dynamic SQL and DBMS_SQL package but theses methodlogy are again should be used cautiously. For example if you are creating a Table you should check first if this table is already exists or not using data dictionary views.