It's difficult to decoy because the missile's processor is programmed to know what typical aircraft profiles look like; for example a transport aircraft has two

It's difficult to decoy because the missile's processor is programmed to know what typical aircraft profiles look like; for example a transport aircraft has two or four propellors with hot leading edges, numerous turbine exhausts and warm leading edges. A flare is a seen as a hot ball, in contrast. To decoy that, the decoy needs to basically _be_ the aircraft.