Transparent code may make finding issues more easily, but it also makes fixing them more easily.
Also it's the most used system used in the world due to sites most using it. So as more desktop users end up using it, problems server managers wouldn't see or deem important now start appearing.
But also, that is considering the statement that security issues in Linux are more common. Are they though? Is it from some technical research, or gut feeling like thinking all airplanes are dangerous when one falls and there's all the noise from the blood-hungry medias?