Way back when Microsoft Office was on the rise, a major selling point was the shared interface, where reasonably possible, across the apps. Word Processing heavily dominates as the most universally used app. A great many people working or recreating day in and day out at a computer with MS Office installed know hardly anything about using Excel or any other spreadsheet. Outside of a college class or similar, many don't use presentation software. Outlook is popular, but came later.
But for those employees, or home users, who only occasionally need to use one of the other apps, it's a nice selling point that, where possible, the menu and icon listings are consistent. Be mindful that way back when, many people hadn't grown up with personal computers. I'm an older Gen. X guy, and my high school graduation present was a Commodore 128 computer setup. In college I was introduced to IBM XTs, and learned on MS-DOS - I recall when Windows 3.0 was big! WordPerfect was the dominant word processor, having dethroned WordStar, and Word was the scrappy new player. WordPerfect was bundled with Quattro Pro and other apps to compete, but by then, the idea that MS Office had you covered and a 'designed that way from scratch' consistent interface caught on, plus Microsoft was such a strong brand, once it got behind something big time, that 'something' was likely going to dominate its niche (just ask Netscape or Lotus 1-2-3).
Some things are different today, but odds are that, from work or school, you already know how to use MS Word at a basic level, and maybe PowerPoint. Excel is a legend in spreadsheets. If people already know how to use Word, and it's readily available, many have little reason to research alternatives. It's also what they can expect to see elsewhere.
Learning a different app makes little sense unless you'll use it enough to make the learning curve worthwhile, and most users aren't power users. For what most people do with a word processor, Word handles it. The original question was why some keep using MS Office on Mac. Because they already know how, it's cross platform and everywhere and the recognized big brand name, and works fine for what they want or need to do.
You can be fairly assured that if you learn Microsoft Word today, it'll still be in use 10 or 15 years down the road. It's harder to assume that with Pages, Ami Pro (yeah, I bought that way back), WordPerfect, WordStar...
I know I'm mixing platforms (e.g.: Ami Pro and WordStar were Windows word processors).