Because they're not part of the HN bubble and genuinely do need an additional program to stop them downloading random infected crap from MediaFire banner ads.
The amount of people who would open funnycat.jpg.exe is much larger than you think. No way are they reading Windows Defender popups with complicated words like "executable", they're gonna click okay and it's up to 3rd party virus protectors to stop that stuff with scary flashy warning popups.
I use BitDefender, not because I'm an idiot as you implied, but simply because according to my research it's more efficient and catches more threats than Windows Defender.
The amount of people who would open funnycat.jpg.exe is much larger than you think. No way are they reading Windows Defender popups with complicated words like "executable", they're gonna click okay and it's up to 3rd party virus protectors to stop that stuff with scary flashy warning popups.