It is more efficient, in terms of cost the datacenter owner pays per unit of heat extracted. Water is cheap in places this is being done, relative to amount of heat vaporization can carry off.

You can certainly argue DCs should pay more for water than other uses, but who gets to decide what is a good vs bad use of water? Pricing in externalities is tricky, and water usage rights are especially complicated. I don't know what a good&fair solution is.

Companies will never price in externalities unless they are forced to.

I would guess that evaporative cooling is usually still cheaper even if you price in externalities. In most of the world there isn't really a shortage of water.