[matt-p]

Yeah the main problem is just that the cheapest/easiest way of getting rid of heat is evaporation*. Once it's evaporated it's obviously not economic to try and get it back at that point. You could have a closed loop system sinking heat into a lake or the sea, even better it could go into a heat network heating peoples homes. All those things are just more 'hard' than ordering a cooling tower.

* In air-cooled datacentres an approach is "direct evaporative cooling". They might take 30°C outside air and spray a fine mist into it, cooling it to perhaps 25°C before it enters the servers. After passing through the servers the air might leave at around 38°C. The water is now dispersed as humidity in a large volume of exhaust air. Recovering that water would require condensing it back out of the air, which means removing huge amounts of latent heat, it would be cheaper to just use 'traditional' compressor based cooling in the first place.

Cooling towers (which are used in many 'ai' facilities) have essentially the same problem. Servers reject heat into a water loop, and the cooling tower then cools that water by evaporating a portion of it into the atmosphere. The water that leaves as vapour is the "consumed" portion. While some liquid water remains in the system and a small amount is discharged as concentrated blowdown that can be treated and reused relatively easily, the majority of the consumed water has been converted into atmospheric moisture as above.