So I know round pipe surface has the least amount wall friction area.
I know you get turbos with round turbine inlets... so why do so many manufacturers make turbos with square turbine inlet flanges instead of round? (T25,T3, T4, T6)

 

im sure someone smarter than me will chime in with a better answer, but a round pipe isnt always the best flowing shape, especially when you take into consideration hot gasses and the fact that they have go into a nozzle shape in the volute area.

 

More surface area and better sealing