... hooked up to the ISS, with humans in attendance to fix anything that goes wrong... not doing very much.
It's akin to the difference between a boat moored up in a port, and an autonomous drone in the middle of the Pacific. Aside from that, satellites have to maneuver in orbit (to stay in the correct orbit, and increasingly to avoid other satellites). Hefting around additional kgs of shielding makes that more difficult, and costly in terms of propellant, which is very important for the lifetime of a satellite.