Why am I not seeing any benefits of parallel execution when parallel clause is used?
You're not seeing the benefits of parallel execution because both insert methods use single-row inserts, hence parallel does not kick in !

Parallel execution is only available for bulk operations. Lots of tiny operations doesn't qualify as a set operation. You're inserting rows one by one here, you should try it with a bulk operation (INSERT /*+APPEND*/ INTO SELECT, CREATE TABLE AS SELECT ...) and see if you can achieve some performance improvement.

You can tell when Oracle does a parallel execution because the explain plan will display parallel operations.

For further reading:


Parallel introduction in the concepts guide
A short introduction about parallel execution by Tom Kyte