Why don't people clean the inside of their computers?

I don't clean it often.It is quite natural to clean the outer of the computer as it is immediately visible.So,nobody will remember to clean the inside of the computer unless they have a chance to open it up for some problems.But as you have told,it is good to clean the inner quite often so that everything works at the right speed.

Almost never do that, probably only when opening the case, but I don't dedicate time to dust my computer.

I am quite aware of the mass of dust that accumulates inside a computer case, and also realise the problems that it can cause. however, like most people it is often too much trouble to remove the case and clean it unless it becomes a necessity. I usually end up cleaning the inside whenever I install hardware because the dust is hard to ignore then.

It can be very dangerous to clean out the inside of a computer. Unless you know what you are doing you can distroy the computer, or shock yourself. Most of the parts continously hold a static charge that can cause damage or shock you if disturbed by bare hands. So if you chose to clean out your pc, be sure you know what you're doing or have gloves handy to touch the sensitive materials.