It's hard to explain what makes an older guy (my dad is 66) stick with Windows. Almost every daily function of OSX is easier, and almost any OS is more secure, but that's just jivetalkin as far as he's concerned. I guess it would be probably true to say that (most) baby boomers are only allowed to learn one OS in their lives. For my dad, that was Windows 3.1. So getting new computers with 95, 98, or XP don't count, because they're the same on the outside. His office computer tech told him to get a Mac, and I tried teaching him how to get around in Linux. But something as simple as the Gnome desktop is totally foreign to him.
Think of it another way. I know that if I want to run the web browser, I have to go find it and then somehow start it, usually by double-clicking. I know that there are icons on my Gnome panel, in my Gnome Applications menu, and in various folders around the filesystem. Plus, if I want, I can open a terminal and get the web browser that way, because I know where it is, I know what it is called (Firefox), and I know how to launch it from a command line. So when I double-click a little picture of a fox circling a planet, I know basically what happens behind the scenes.
Contrast that to my dad, who only knows that if he double-clicks that picture on the desktop, the web browser magically appears. If the desktop icon was gone, he wouldn't be able to open Firefox. Even though it's in the Start Menu - well, maybe if he fucked around with it for awhile, he might get it to open that way. And my dad knows that he's not really creating magic by double-clicking, he knows that the web browser (whatever its name is) is a program that he is running -- but that knowledge is divorced from his hand moving the mouse over an icon and clicking twice.
It's the same with your grandmother in the old Cadillac - she knows how to work the steering wheel and pedals, and she may even know that the car needs gasoline to run. But she doesn't know why the car needs gasoline to run. And she really doesn't need to in order to drive. When the car doesn't work as advertised, you can see these people's thought processes: "the key is in the hole, and I turned it, and it doesn't start - is it the right key?" Same with computers: "I double clicked it and nothing happened, better do it again".
Here we go, I just thought of the perfect way to explain it. These people are unable to see below their current level of abstraction. They're so engrossed in the user interface they can't see the machine. And because of their ignorance, they mistake the user interface for the machine.
As long as this sort of metaphysical engineering continues, there will always be a market for neighborhood mechanics and Microsoft Windows. Consider yourself fortunate that you have seen beyond the surface down to the next level or 2 - because it's clear that not everyone can do that.