Directors, financial people and managers often aren't IT guys, will willingly take anything Microsoft or Apple throws at them if it's going to make their lifes easier and beg their IT departments to install it on their existing intrastructure. I mean, every once in a while at one point you hear about hackers going through iCloud to leak celebrities' nudes it's laughable. It may be secure, but it's open way too easily to social engineering: you just ask some guy who know nothing about IT but have a lot of power at a big company nicely and bam, you're in, stole his Office account and/or the company's domain names.
It's kinda weird, and I'm pretty sure the big ones are doing everything possible to limit hackers, but it's always going to happen, sadly. Nothing is infallible.