Why are computers so complicated?
The main reason why computers are complicated is because they are really optimized to deliver the maximum speed. There are a lot of places in a computer where information is stored:


processor registers: here the speed of memorization is really high, but this memory is very expensive;
random access memory: slower and cheaper then processor registers, but more expensive and faster then hard disk memory
hard disk: one of the cheapest and slowest kinds of computer memory


There are also cache mechanisms in between of these levels.

The first definition of computer is that of a person computing some arithmetical operation. So a person could be seen as a very familiar kind of computer, or a biologically very complex one.

Not all computer models are complicated. It has been proved that the following models can compute the same types of problems:


Turing machines: just a tape and a finite state automata;
lambda calculus: few definitions are enough to achieve a system to manipulate symbols;
Godel's general recursive functions