It's not really necessary to go 4-slot air cooler when you've got water cooling.
For datacenters it would make more sense to make it all water cooled though, since you can likely have one central loop connected to an HVAC unit or something.
Ignoring that, putting the radiator anywhere other than the precious motherboard real estate has a lot of value. It's such a waste to hang a 10 pound piece of copper over all of those fast, unused PCIe lanes.
Didn't Gamer's Nexus or LTT show a board like that a few years ago?
If you produce a great solution, but everyone has to source or build a custom case, you probably aren't going to sell many.