Why RDBMS is not suitable for Semantic DB

Since the day I started working on Semantic DB, I found that there is a need for a database which is very flexible and not rigid like RDBMS. I know many people will argue why I say RDBMS is rigid. Here is why I see RDBMS not working for me when it comes to store objects in semantic way.

1. It has a very fixed Structure. Define once and let all the objects fit into the structure.

2. The data is stored in a contiguous block in the database.

3. The chances of duplication is higher. I mean to say that a value cannot be uniquely identified in the system.

I guess I am looking at the wrong technology to solve my problem.