Why isn't there a final permutation on SPN-based ciphers?
Because a fixed and published permutation is known, linear and ever so reversible. It's very much like a bait &amp; switch move, but without the switch(substitution). Security is built piecemeal from units of permutation &amp; substitution(rounds). We can undo a singular publically published permutation, and it doesn't take any notice of a key(as it's fixed), thus it adds nothing security wise. Nobody really gets confused and the diffusion effects can easily be reversed.

As a kiddie, slightly similar example, think about ROT13 so called encryption.