This is actually an excellent question.
The fast answer is that we are not good enough yet to make them simple.
As Pascal said in his 16th Provincial Letter : "Je n'ai fait celle-ci plus longue que parce que je n'ai pas eu le loisir de la faire plus court". In English: "I wrote a longer letter, because I did not have time to make it shorter".
Similarly, computer scientists did not yet have time to make computer easier to use. Though they have done much better than you think.
But there are really many ways to understand your question:
one concerns the complexity computer hardware, and that was answered previously
by Vitalij Zadneprovskij.
why is it so hard to use computers as a programmer?
why are computers complicated to run my favorite game, or to use such and such application?
The second question is easy to answer: you just look at the kind of task
that you expect from computers. Some of them require considerable work that can be incredibly complex,
like running a nuclear power-plant, or like flying an airplane so that
you have simple commands even for complex and unstable situations (some unstable planes cannot be flown without a computer), or
driving a car automatically, and many other problems that you most
likely would not be familiar with, some of them being purely
mathematical. Computer can manage tasks that would swamp any human
mind by their complexity.
We are tranferring to computers all sorts of complex tasks that were
suppose to require (more than) human intelligence and abilities. It is not
surprising that you end up with very complex devices, very complex
machines, complex at least for the people who make them and are
responsible for setting them to these complex tasks, whether through
hardware or through software.
The point that was made by Vitalij Zadneprovskij is that the hardware,
the physical part of the machine could be actually very simple, but
that complexity comes from the need to make if efficient at a great
variety of tasks, and to connect them to sensors and effectors, which
are physical devices. This means that much of the ability to handle
complex tasks comes from the software. Very elementary models of
computers have been devised by mathematicians and logicians. They are
all able to perform the same computational tasks as highly
sophisticated modern computers ... but not necessarily with the same
So we have two kinds of complexities at work. One complexity is due to
the efficiency requirements of the hardware, and the need to connect
to various devices. The other comes from the very complexity of the
tasks we set them to, which is largely to be found in the software.
But you want simplicity of use.
For example, you do not need to know much about the inner working
of your car: no mechanics, no thermodynamics, no chemistry of
combustion engines, no differential gears (that helps taking turns),
etc. Most people do not know any of these things, and yet they can
drive their cars.
So you would expect the same for lots of other machines, including the
computer. But your home computer is complex.
Well, the problem is that a computer is not analogous to your car, but
to the engine of your car. The engine in your car is a very complex
machine, but it is under the hood, and you do not really have to know
about it when you drive.
As it turns out, you can very easily use a lot of computers with two or
three buttons as you are asking. When you drive your car, you have a brake
pedal (that is just one button) that runs a computer that does the
braking, much better that you would, using the ABS technology, giving
better braking by relaxing the brakes for tiny periods. Then, when you
accelerate, I guess that another computer will increase the fuel input
in just the right way. Then, on my car, I have a no button computer
(well there is a button to turn it on) that will chide me if my wheels
step on a line without the car lights blinking to indicate the move.
I also have a computer in my washing machine, but I find it a bit
harder to use. Then there are computers in the TV. Well to be honest,
we are surrounded by computers that are so easy to use that we do not
even know there are there. That is why you are not even aware that
computer are so easy to use. It is so easy you no longer notice them.
So why is it that you still have a home computer that is hard to use?
The main reason is that you have closer access to various functions, so
that you can put it to a wide variety of tasks. We are not yet very
good at preserving the flexibility while making it easy to use, though
that is improving all the time. See for example how easy it has become
(well, most of the time) to use a tablet. Smartphones are more
complex than phones, but do so many things.
We are not yet good enough to make computers very simple, while preserving their innate flexibility, though we made enormous progress. But many computers have become (at the expense of flexibility) so simple to use as to be invisible.
You are right to ask for simplicity. But as Pascal might say today:
simplicity requires considerable work that can be very complex. The
same Blaise Pascal who invented one of the first arithmetic machines,
and gave his name to a programming language.