MaJoRa. There are a lot more reasons that you have to deal with mostly problems dealing with IE7 and 8. It definitely is not only IE's problem.
1) The biggest part of the population uses IE, because it is standard in windows (the most used OS in the world). That part is even larger among the people that need IT technicians to solve their problems.
2) Most viruses and problems are made for IE.
3) The people that need IT technicians to solve their problems are the people that do not know how to work with the PC in the first place. They visit websites that are unsafe and do not know what not to do and not to click on spam.
If people don't bring in the problems they wouldn't need anything solving also.
I agree to your first point, the vast majority of the people do use IE, because it is a standard on every machine. I think this sort of ties in with your third point too. These are also the people that do not know how to solve their own problems, and cant use a computer in the first place, they click on unsafe websites and do things they shouldn't. Though, you could also argue that those who do know what they are doing are using browsers such as firefox, safari and chrome, and that is why they are less likely to (though then I would have to go into detail about the security flaws, which I can't at this time).
You're second point however, I also agree with, this is for several reasons. The first reason is that it is the most common browser, it makes sense to problem a virus for it. The second is that it does have huge security weaknesses, making it easier to program them for, and these weaknesses are better known. But the reason for a lot of these flaws is because the software is not open source. Their are not enough people who are working on it to get rid of these problems. With open source browsers, you have people attempting to heck them all of the time for fun or to prove a point, and when this happens the weaknesses get resolved faster.
Anyway, I think we should continue this argument on a discussions board or something, would be far more fun, its like taking over some ones topic, since use debating this doesn't really help him get rid of the annoying error.