Maybe 5-10 years ago companies didn't realize it. I think every major corporation has started to consider that kind of scenario now. Smaller companies may still delude themselves into thinking they aren't targets, but they probably are much less capable than big companies of investing in the required preparations to prevent those attacks.

The "black swan" events aren't costless to mitigate. In fact, they are extremely expensive. And just like there can be a black swan for cyberattacks, there can be a black swan like a terrorist attack (like companies in World Trade Center Towers 1 and 2), a massive natural disaster (think businesses in Puerto Rico right now), massive electrical outages (say an EMP or simply just Enron malfeasance), etc.

Spending to protect against and mitigate all possible black swans approaches infinity and doesn't every pay off until the extremely rare but extremely impactful event ever happens. And in cyber security, you never know because your defenses prevented it before it ever started.