ceph(and the others I assume) solves the same problem that raid solved, but using the network as a transport layer(rather than the computer for raid). this generally means it is very easy to scale.

So the answer to your question is that your question was wrong, you are comparing different layers.

you can actually access ceph over nfs, however usually you will try to use a ceph aware access method, ether the ceph filesystem driver, or the S3-like library depending on what your application wants.