"High emissivity, high temperature" sounds good on paper, but to create that temperature gradient within your spacecraft the way you want costs a lot of energy. What you actually do is add a shit load of surface area to your spacecraft, give that whole thing a coating that improves its emissivity, and try your hardest to minimize the thermal gradient from the heat source (the hot part) throughout the radiator. Emissivity isn't going past 1 in that equation, and you're going to have a very hard time getting your radiator to be hotter than your heat source.

Note that KSP is a game that fictionalizes a lot of things, and sizes of solar panels and radiators are one of those things.

How many times can you do that?

Consider your own computer... how often does it get hot under a regular load and the fans kick on? That "fans kick on" is transferring the heat to air and jettisoning it into the room... and you're dealing with 100 watts there. Scale that up to kilowatts that are always running.

There is a lot of energy that is being consumed for computation and being converted into heat.

The other part if that is... its a lot easier to do that transfer heat into some other material and jettison it on earth, without having to ship the rack into space and also deal with the additional mechanics of getting rid of hot things. You've got advantages of things like "cold things sink in gravity" and "you can push heat around and sink it into other things (like phase change of water)" and "you don't need to be sitting on top of a power plant in order to use the power."