Why maven do not "add" all dependencies? (on spring example)

I add sprig dependency to my maven pom.

<dependency>
        <groupId>org.springframework</groupId>
        <artifactId>spring</artifactId>
        <version>2.5.6.SEC01</version>
</dependency>

I "run" maven to download all dependencies. In spring pom file (from local .m2 repository) I can see many dependencies (about 20)

        .....
     </开发者_开发问答organization>
 <dependencies>
    <!-- External Dependencies -->
  <dependency>
   <groupId>aopalliance</groupId>
   <artifactId>aopalliance</artifactId>
   <version>1.0</version>
   <optional>true</optional>
  </dependency>
  <dependency>
   <groupId>asm</groupId>
   <artifactId>asm</artifactId>
   <version>2.2.3</version>
   <optional>true</optional>
  </dependency>
        .....

But when I collect all libs by Assebly plugin or list all of them using "dependencies" task, I see that maven "use" only spring and commons-lang. No any others.

I just want to know whats happens or what I am doing wrong? Why maven do not "add" all dependencies from spring pom?

---

These dependencies are marked as optional (<optional>true</optional>) in the pom. Hence they do not get included.