Why are single-trip flights so expensive?

Does anyone know the (or a) reason why single-trip flights (in particular Europe <-> USA) are so ridiculously expensive? In my case, I want a trip starting in one European city, going to the USA, returning via Iceland to another European city. Three segments, four different cities. Prices I can find are around $5000.
Looks like I’ll have to end up with going back and forth between a few cities more than I like, to keep the price reasonable.