It all basically boils down to: in order to dissipate heat, you need something to dissipate heat into, e.g. air, liquid, etc. Even if you liquid cool the GPUs, where is the heat going to go?

On Earth, you can vent the heat into the atmosphere no problem, but in space, there's no atmosphere to vent to, so dissipating heat becomes a very, very difficult problem to solve. You can use radiators to an extent, but again, because no atmosphere, they're orders of magnitude less effective in space. So any kind of cooling array would have to be huge, and you'd also have to find some way to shade them, because you still have to deal with heat and other kinds of radiation coming from the Sun.

It's easier to just keep them on Earth.