The big server market uses a well-worn maxim in the market: good enough. The economics of the situation dictate that R&D must be stopped when the returns do not outweigh the investment.
Big iron in the 70's used watercooling, and it was generally despised because of the complexity and (therefore) low MTBF. So, the big server market uses air cooling now. It is much more reliable and it is also cheaper to produce. Chips are fast enough for the server market such that the decreased reliability and increased manufacturing costs do not justify watercooling.
Now, if you want to throw out cost, you end up in situations where you use phase change cooling and live with a potentially inferior junction. It is cheaper to throw another stage on the phase change cascade than it is to perfect the heat exchanger. Low production volume. If you are interested in a chiller manufacturer, look at Temptronics. They produce chillers for the semiconductor test environment. They use phase change to cool a liquid and then route the liquid to the heat exchanger on the backside of the wafer.
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