Devastatingly effective spear phishing
The short version is, if an attacker is going for you specifically, they can do enough research to craft an email and attachment that you are almost certain to open. The success rate against even very paranoid and sophisticated users is shockingly high.
In Bruce Schneier’s blog post about this he quotes Brian Snow, former NSA Information Assurance Director. “Your cyber systems continue to function and serve you not due to the expertise of your security staff but solely due to the sufferance of your opponents.”