They would need a breaking motor and possibly attitude control. Also they’d need a way to grasp their payload. All this would eat into the max payload to LEO for the rocket as a whole (by quite a lot). So… maybe for flights that are well under max payload, like the ISS resupply missions (which usually are about 6 tonnes below max).
I also have to question whether the fairings can survive reentry from orbital speeds (especially when carrying extra mass). Normally they are dropped at 2500 m/s, which is quite a lot less energy than the 7000-8000 m/s of orbital speed.