Look up "herd immunity." No man is an island.
Additionally many do inadvertently infect their Mac. Hence, the anti-malware programs.
"herd immunity" does not apply here because we are not talking about malware that can spread between Macs, or even Mac malware that can spread to Win PCs. I could ask you to lookup "natural selection" however that doesn't seem to apply to humans any longer as they do really stupid things every day like self inflicting infections on their otherwise extremely secure computers that have many, many layers of built in malware protection.
I will say this over and over but installing active malware scanning software on your Mac almost certainly makes your Mac MORE vulnerable than is was without the software. This type of software is currently "snake oil" on a Mac. It prevents nothing more than the Mac OS already protects against, yet integrates itself into the OS at a low level where it can introduce vulnerabilities that were not there in the first place.