Right, but running redis in that manner is far more operationally-intensive than running several nginx worker threads.
nginx runs workers by default, which (I believe) can be tuned by a couple config options.
To run multiple redis instances as a part of the same cluster, you need a way to shard your data (which you have to reason about client-side), you need separate config files, data directories, etc. for each instance. It's a huge pain.
