It's definitely a solvable problem. But it is a major cost factor that is commonly handwaved away. It also restricts the size of each individual satellite: moving electricity through wires is much easier than pumping cooling fluid to radiators, so radiators are harder to scale. Not a big deal at ISS scale, but some proposals had square kilometers of solar arrays per satellite

That exactly. It's not that it's impossible. It's that it's heavy to efficiently transport heat to the radiators or requires a lot of tiny sats, which have their with problems.