Why create such a complex cipher?

[Rafal]

But why the strange alphabet?

Well I don't have a good answer for that. I am suspecting that symbols in Voynich words are kind of numbers but I don't have any proof.

But I would say that ciphers changed through time. Modern ciphers are most often made of standard letters and numbers. Old ciphers used a lot of fancy, invented symbols. If you were making a cipher in the 15th century, you would probably feel "obliged" to use some symbols created by yourself