Because...
thats almost unbelievable to me...No firewall, antivirus, or antispyware...If a PC didn't have all of that it'd be destroyed...I've had many of my friends come to me with a PC computer problem...their computers about to die...I find that they have just the windows default firewall on *always a mistake*, I install ad aware and run it to find hundreds of spyware files...I install a anti-virus program to find many virus's, including trogans...
I don't understand how a mac could avoid all these issues without an extra program...
Most viruses/worms/trojans/spywares are built to work on pc's, and won't work on macs simply because of the different os.
My common sense tells me- the market share is sooooo heavily saturated with PC and Windows, that malicious code writers are going to write their spyware and viruses for PC users to get more bang for their buck. As mac's share (and popularity) grows, we're starting to see successful worms and hacks for the mac. And don't be fooled- OSX is not some beacon of security. And if tomorrow we wake up and Apple's share is equal to that of PC- unplug your ethernet cables, because the code will be coming.
Search "security" on mac rumors search and dig down a little- you'll see articles- sometimes involving Secunia, a security firm that believes OS X is fairly insecure and posts flaws occasionally, and can read about it.
My final comment: OS X markets itself to be veeeerrry secure, and I fear that might motivate code writers to make viruses just for the "arrogance" of mac marketing their "wonderful" security. Maybe I'm paranoid though?
Edit- But I do love mac, and I do feel rather secure... for now.