Nah, it's because of the volume.

You don't cool down hot water with the same amount of cool water. You use a shit ton of cool water, because the larger the difference in temps the faster the heat exchange.

So the discharge isn't water that's really hot. It's just warmer than when it went in.

Maybe 5-10 degrees, which is enough for a negative environmental impact if constantly discharged into a lake/ocean/river, but not hot enough to be good for anything.

They could do large underground reserve for cold water, cool their servers with it, then dump it into a second tank that eventually cools and is added to the reserve. It's not complicated, but it is a huge upfront cost.

Companies aren't going to do it when they can pay a fraction of the cost even tho it fucks over everyone else. This is capitalism, we need regulations forcing them to do the right thing over the cheap thing.