I think cause the human body is more susceptible to being nuked? After all, presumablly the AI would reside in a hardened bunker and therefore immune to being nuked.
If the AI launched neutron bombs, then it would be even better...
thats the thing. in t3 it says skynet wasn't a computer in a bunker but all the computers in the world. even if it infected an updated computer system unlike those old ones shown in the bunker john and his future wife got tricked into, skynet would still destroy vast parts of itself.