Why did it become so much more expensive to start a university?
Besides all the factors that the other answer already lists, the elephant in the room is that KAUST has explicitly been designed to be a world-class university (rather than organically growing into one, as was the case with your other examples).

In short, it is not so much more expensive nowadays to found just any university - in fact new universities get founded all the time, virtually always spending considerable less money than $10 billion - but founding a university with the expectation that it will compete with the best in the world in a very short time frame is expensive. You need to compensate for all the natural growth in budget and prestige that established universities had over the last one or two centuries.