Remember, most people do not view the computer as a world to be explored. It is merely a means to an end. So they learn the five steps they need to follow, and if they can't do them, they get stuck. "I hit Alt+Tab like I always do, to switch to another program, but instead of switching, this strange window showed up. Help!" Or "I print the document by clicking the Actions menu and selecting Print, but now there is no Actions menu. Help!" Changes to the user interface also mean that screenshots need to be re-taken and training materials reprinted.
In a sense, people act computerlike when they are in front of a computer!
I behaved the same way when I was working at product support earlier this year. The Product Support division has a system for tracking calls and issues, and we were given a handout with instructions like, "To transfer an issue to XYZ, click Transfer, then select XYZ from the list." Now imagine if the Transfer button weren't there any more (maybe it got moved to a sub-dialog) or if XYZ was no longer on the list (perhaps it got combined with QRS). I would have been stuck. I'm not going to go hunting around looking for the Transfer button; if I click the wrong button I might create a corrupted record in their database and create more problems than I was trying to solve. So when I couldn't follow the instructions, I called for help.
There are people who argue that, "Well, in order to use a computer, you should be required to learn how a computer works." Pshaw. I drive a car and yet I don't know how a carburetor works, what the optimum fuel/air ratio is, or even how many cylinders my engine has. I don't care. All that matters to me is that I step on the pedal and it goes. We don't require people to be auto mechanics before they get a drivers license (at least, not in the United States; other countries may be different). Why should we expect them to understand how a computer works before they are allowed to send email?