You can't pass file descriptors over sockets. You can't do RPC over pipes. Anonymous sockets can only be shared with children. Named sockets require write permission and a filesystem. SysV is prone to resource leaks.
Binder has authentication, shared memory, reference counting, weak references, dead object notifications.
The descriptors and authentication part is key, as it allows Android's trusted system_server to open a device or socket for an untrusted process such as an App if it has user permissions. The receiver of a binder request can check the requester's PID/GID and with confidence from the kernel.
Most importantly, I don't think it's healthy to think of POSIX as being done. Some experimentation and discovery of better APIs and better systems should be encouraged.
gah, I'm driving myself crazy; I swear I saw somewhere a diagram of the Taxonomy of Unix IPC Mechanisms, with most of them crossed out and replaced simply with binder, please help me find it!