Why don't companies fill these aio's with non conductive water? This isn't the first time I've heard of a aio leaking and destroying someone's computer.
I'm thinking off the top of my head that distilled water permanently sealed in a loop of metal and plastic would become not only conductive over time but risk growth and corrosion/galvanic activity.
A custom loop can be filled with whatever you like, when it corrodes your metal parts, clogs the radiator and starts going green from algae you can flush it, swill the rads with vinegar and start again.
From the POV of a sealed loop manufacturer they want something which won't cause any issues over the life of the unit when permanently sealed in.
The Corsair H50 I've had since 2009 is filled with ethylene glycol.
If your loop custom or sealed leaks you're screwed, just have to consider the risks when you do it.