the minimum requirement.

Well yeah but...

That would require a different way of cooling that water in that closed loop, which would increase power consumption by up to 100% as the servers used power to heat the water, and to cool it you'll need at least the same amount of power to cool the water back to what it was before. Add in losses, and you'll at least double the power usage.

So far it's dumped into the environment with the reasoning that that sink is so large that you can keep dumping into it without any side effects; nature will cool down the water for free.

That being a big fat lie has already been known since we found out that nature won't magically absorb garbage either, of course, but that is the reasoning.