My last virus was in the mac os 8 days. Mac viruses back then tended to be additional resources added to the application, usually in window resources or MDBFs. Nowadays inserting code into a mac app isn't nearly as trivial, and I think that helps
But really, viruses aren't all that big of a deal nowadays. There's no money in it, and you're far more likely to interact with people over the internet than you are with physical media. Back then people were making malware for lols and that's about it.
Scareware and botnets are the current threat. Plenty of money to be made in both arenas. Spreading over the internet is so much more efficient, and it removes the automated computer element to a large degree. Instead of relying on security holes in the os, it targets the user as the weakest point in the security.
There's nothing better than Money to motivate and add sophistication to illegal activity.
