Why do you thik there's no antivirus? There are antiviruses that run on unices, like ClamAV for example - it's open source, they can include it for free - maybe they did.
ClamAV for Linux et. al. primarily targets Windows threats. It's primarily used to filter e-mail or scan SMB shares, not look for threats that might affect the machine they run on, so it'd seem a very odd choice to include.
