Because my Mac never shows any symptoms of it (it is a piece of cake to watch all the active processes in OSX and see if there is anything new). Runs the same, all the time. Any malware I might have passed on came from a compromised PC in the first place (and didn't affect me). Not my problem...

All I can say is MY experience. 15 years and nary a hiccup. No security software. YMMV (especially if you have the Intel Mac boot into Windows!). Any PC users not IT specialists able to say that?

I doubt it.