To make it short: Because they can. As others have pointed out there are taxes, regulations and so on, but the main reason (in my opinion) is that they still make a very good profit in Europe, so why should they reduce prices? Apple is a business and businesses will do almost anything to make more profit (that's the reason they exist, nothing wrong here).
p.s.: Steam has a similar policy and one could note that Steam and Apple share a common trait: No real alternative.
If I remember correctly you could buy with dollars at Steam as an European customer until one year ago, since then this isn't possible anymore. Was this change forced by the publishers?
Anyway, thanks for the hint. Pricing of Valve games is indeed near parity, so you are probably correct regarding publishers/Steam.
p.s.: Steam has a similar policy and one could note that Steam and Apple share a common trait: No real alternative.