Because it's been a problem since 2004 and earlier, actually
The bloody 'iLoveYou' virus from 2000, Code Red and ilk from 2001, and so on highlighted grave weaknesses in scanners that have existed to this very day. Never mind 2004.
You can blame McAfee for spearheading that, even back in 1999:
The market for profile-based detectors dried up ... because software reviewers recommended signature-based scanning to the exclusion of all else. That's right: journalists with no expertise told everybody to use insufficient virus detection methods.
Profile-based utilities shriveled when John McAfee released a signature scanner called VirusScan. [...] Signature methodologies complement profile methodologies, and vice versa — but McAfee told the media his scanner superceded generic detection. Reporters wrote countless stories saying we needed only one product: VirusScan.
McAfee set themselves up for this failure at least thirteen years ago.