[AnIdiotOnTheNet]

Because real-time malware scanning causes way more trouble than it is worth. It adds latency to every file open on the off chance that it might contain some kind of identifiably malicious code. That might be mildly useful if I had any faith it could actually detect the majority of malware, but I don't. In fact, the only things I've ever seen it identify reliably are any kind of piracy or forensics tools.