There is a move to put more of the switching and routing logic in software (google for openvswitch if you're interested) but part of the problem is that general purpose hardware doesn't stand a hope in hell of keeping up with interesting network data rates. You absolutely need to be doing a fair portion of the work in hardware, the CPUs just coordinate it.

It's probably worth reviewing this article on Intel FM6000 SDN to get a glimpse into the complexity involved.

http://www.intel.com/content/dam/www/public/us/en/documents/...