Viruses are free.

Viruses don't take up most of your hard drive.

Viruses don't need 80 megs of RAM.

Viruses don't have major bugs.

Viruses don't have three different sets of documentation.

Viruses don't leak info to the press about the upcoming Jerusalem 95, to keep people from switching to Michelangelo/2 Warp.

Viruses aren't on every computer.

Nobody cares if a virus turns out to be 16 bit, even though it is advertised as 32...

Viruses install themselves !