Strange casting

Now I don't know the first thing about film casting, so mabey someone can help me out here...My question is, why did Ridley Scott cast Idris Elba (an Englishman) to play Janek (an American)......cast Rafe Spall (an Englishman) to play Milburn (an American).....cast Noomi Rapace (Swedish) to play Shaw (British).......and most confusing of all.....cast Guy Pearce (who is in his early 40's) to play Weyland (who is approximately 85 - 100 years old)?......Don't get me wrong, I thought the actors gave excellent performances (considering the garbage script the had to work with)...but it seems kind of unnecessary to cast actors to play characters from different demographics, especially considering the number of actors out there who would've been much better suited for the roles........(I read somewhere that poor Noomi had to Lear English and develop a convincing accent just to get the part.....see what I mean by unnecessary)..........(btw: when I say there are actors out there better for the roles I mean like getting an old actor to play an old guy.)