You are thinking of signals as just the lights telling the driver what to do, but the signals used in the subway are much more complex and include the hardware

You are thinking of signals as just the lights telling the driver what to do, but the signals used in the subway are much more complex and include the hardware to know, for example, whether another train is on an upcoming stretch of track, or how a switch is set up ahead, which affects what tracks the train is routed onto. These aren't just electronic systems, they include mechanical hardware to detect the necessary information. The design of some of the signals still in use basically hasn't changed in 100 years!