Supply and Demand. More demand is better terms for stock sellers, i.e. companies. And their investing would probably completely skew the market. So it's hard. Investing directly might put them at risk of some kind of oversight as well.

And they already have a lot tied up in those kinds of investments anyhow. They own a lot of real estate around the world as well.

It's hard to find places to park that amount of money esp. if there are political considerations. Nobody is worried about taking money from the Norwegians ...