Waterproof belay jacket ?

I’m curious why belay jackets arnt more commonly waterproof. Imagine you are mid way on a wet pillar. Wouldn’t having a waterproof belay jacket be usefull ?

Cost, complexity, weight and packability, versatility. 

It's certainly possible, it might just be too much of a niche market. For most users, synthetic fibers able to insulate when wet work decently enough. How many people would be willing to spend essentially the money of a good hardshell plus the money of a good belay parka on such a specific jacket? 

Because you don't belay halfway up a pillar and definitely not under a dripping wet section. Find a sheltered stance to belay from, away from the falling ice. 

Eh, there’s plenty of mostly waterproof belay parkas available with gore windstopper shells. 

Mountain equipment Exo has a taped shell, phD makes most of their jackets and some sort of waterproof taped material

As Eli mentions, waterproof belay jackets are plentiful, any Infinium or Windstopper belay jacket such as those produced by mountain equipment are functionally waterproof with Windstopper having a hydrostatic head of up to 28,000mm. the difference there is the lack of taped seams. Many jackets that use materials like Pertex endurance(I think it is called Quantum Pro now) are also functionally waterproof having a high hydrostatic head of about 4000mm and many generics that brands use also have a thin PU layer to improve HH. Waterproof breathables are in this case preferred as studies have found that coated Pertex and the equivalents have worse breathability than waterproof breathable membranes.

There is the question as to the benefit of this though, synthetic insulation doesn't lose much insulation when wet, if you're ice climbing in wet conditions your action suit should be able to handle that moisture fine and you shouldn't need extra waterproofing from a belay jacket, adding waterproof layers makes the jacket more difficult to compress, stops you from drying out and is more likely to cause condensation on the inside of the jacket reducing breathability and limiting your ability to dry out at the belay and in a sleeping bag. If you can find a jacket that uses a hygroscopic PU waterproof breathable membrane (Pertex shield and most generics) instead of ePTFE (Goretex and e-Vent) they perform better in cold conditions and are less prone to condensation.

You can read about Mark Twight talking on this topic in Extreme Alpinism and it is available for free at https://archive.org/details/extremealpinismc00twig/mode/2up