It's not like my kitchen only fits Whirlpool or my garage only fits Honda.
Chances are they have software installed on their machine. OSX has a vibrant ecosystem of independent developer and thus a pretty large number of bespoke good-quality software which might / would have to be replaced. Especially as cross-platform software tends to integrate less than well with the platform.
That goes double for older mac users, which have a higher tendency to use native software.
And of course one needs the time to adapt to different paradigms, shortcuts, facilities, … once again especially for older users of the platform for whom this becomes second nature.
Incidentally the points mix, the second one drives the first, I regularly notice cross-platform software which doesn't respond properly to Cocoa's text-movement shortcuts (even a simple C-a / C-e).