Yup, phase changing. Water in vacuum has to touch the heat source, evaporate, travel to the cooler side, condense and start over. As soon as your heat source prevents the cooling from happening, the cycle stops and it over heats. Like there's a point at which the water can't cool the source because its overwhelming.

It doesn't work for a lot of things, but your PC is basically a couple of flat panels where a lot of heat produced is localized to the GPU and CPU and it is potentially very easy to maintain a directed airflow close to them. A heatsink is basically good enough for that, and almost every GPU incorporates one, even the 5090's, even though the airflow design usually sucks for all of them because of how generic it is. Speaking of car engines, you can go a step better and precisely control the airflow to get great cooling results just with air cooling: https://www.youtube.com/watch?v=cehXZftIYok

I see your point, but if it works nearly as well, why go for a costlier solution that requires more maintenance and lasts less when it doesn't require it?

I feel the need to point out that water cooling has an upper limit as well. If you get things hot enough, the water is going to flash into steam, at which point it’s going to decimate whatever system it’s in.

You can add additives to prevent that, but at some point it’s no longer “water”.

Reading this back, I suppose this is pedantism. But still.