Because it's universal, and many businesses and educational institutions use the Microsoft Office suite. In fact, my college offers a free Microsoft Office 365 subscription to students, and I found it pretty handy, especially the OneDrive feature for storing copies of my documents for schoolwork and such. Plus, said college also offered classes in Microsoft Word, Excel and PowerPoint, and they are typically a requirement for most degrees. Going for an Associate's in Computer Information: User Support, I had to take those, along with a business data entry course that utilized Microsoft Access. Most of these classes, in the computer labs we took them I could start the files in the Windows versions on their Dell PCs, and finish up the files and any other homework on the Mac version of Office on my Mac Mini! When following the assignment instructions in a textbook or whatever, it was usually easy to follow along in the Mac versions, though sometimes I'd need to look something up to see how the Mac equivalent would work. (Though for the class using Microsoft Access I'd use the Dell PC I set up in my room alongside my Mini for running Windows programs to do so, along with there being a few features in the other apps that required me to use the Windows versions too; there are advantages to me being a cross-platform user!)
Also, me and my family have used Office for such a long time, since we're primarily a Windows family (and even though I'm the only Mac user in the family I still like to use Windows for certain purposes), and I like the aforementioned universal appeal of Microsoft Office. Since I graduated, I subscribed to one of the more affordable Office 365 plans, one that pretty much lets me only use Word, Excel and PowerPoint, because I find those to be the only Word apps I'll really need to use in my life (and even then, it's mostly just Word that I use!)