It's because often your software engineers know software development and just enough administration and operations to be dangerous, and your administrators know

It's because often your software engineers know software development and just enough administration and operations to be dangerous, and your administrators know operations and just enough software development to be dangerous. Letting them wade into others responsibilities often introduces a lot of that danger. System administration is an entire discipline with a history and way of thinking and approaching problems at a systems level. Software engineering is the same, but with an approach that often emphasizes handling problems in the application. Letting solutions be put into applications that are best handled at the system level (where system isn't just an OS, but might be a complex network of them), or put into the system when it could easily be handled by the application, caused inefficiency and problems. The solution is to either hire only very accomplished people that can do both and make the right decision of where to put solutions, or hire people that can do one or the other and put them together with a few (maybe one) person that has a good grasp of both and can make executive decisions when the right time and place is for specific solutions (an architect). The latter is easier to hire for.