Why are plan-driven (rather than agile) processes used in systems engineering?

Why are plan-driven (rather than agile) processes used in systems engineering?
Ans: Plan-driven processes are used in systems engineering because different parts of the system are being developed at the same time. For systems that include hardware and other equipment, changes during development can be very expensive or, sometimes, practically impossible. It is essential therefore, that the system requirements are fully understood before hardware development or building work begins.
Reworking the system design to solve hardware problems is rarely possible. For this reason, more and more system functionality is being assigned to the system software. This allows some changes to be made during system development, in response to new system requirements that inevitably arise.