Well I've realised I have been never hit by a car so... I don't need to watch by my sides when I cross the street...
The concept is not properly announced. Antivirus is needed in almost every case. Even Windows Defender works when you're an advanced user. Nowadays, with the huge amount of malware coders, new tactics and techniques evolve and pop up on the security industry that can trick even the brightest IT nerd. Some years ago, nobody would suspect you could get infected by only watching an ad banner, not even clicking it, and here we are, totally possible -and common-.
My recommendation is: always use an antivirus even if it's basic in case you're an advanced user, and always complement with signature-less software.
A good combo can rely on Windows Defender and OSArmor, if you know what you're doing online. Can't think of lighter combo.