Why are there different GlassFish v2 downloads for AIX, Solaris, Linux, and MacOs?

Because platform-specific differences and requirements need to be addressed. Although GlassFish is based on Java, there are still some platform-specific utilities such as dynamic link libraries for security, CLI, and so on.

Also, for certain operating systems, it is necessary to use the JDK that comes with the Operating System. For instance, Mac and AIX have their own JDK. Compiling with those JDKs is required to ensure that large and critical pieces of software such as GlassFish do not have problems on those platforms. Thus the need for platform-specific JAR file downloads.

You may also look at the community-provided IzPack installers for GlassFish. They offer a single artifact download experience. Note this is not a Sun-supported installer.