Because liquid cooling is only a marginal benefit for current hardware, hardware designed to be air cooled. If liquid cooling becomes a norm we should start seeing hardware designed with the expectation of liquid cooling. That could mean greater component density and greater power concentration --> greater speeds. One wonders what CPUs would be like if they were designed with the expectation of liquid cooling, water within a millimetre from the heat source rather than the metal heat spreaders that currently top most chips.
How many fans are in a modern server rack? Liquid cooling could theoretically replace all of those with one moving part, one pump impeller.
Or maybe more than one impeller, because at least with the fans, cooling continues if there's a failure in one of them.
And this problem has been there since Day 1 ( For well over a decade ). And no one seems to have solve it. It make sense if you treat the whole Rack as a single entity with water cooling. But for individual server I still dont understand how and why liquid cooling could benefits.