The problem with stock is when you buy stock you have to sell stock. So you have to have a price and you have to have a market and your point of connection with the enterprise you are investing isn't the customers and the revenues, it's what other investors think their share price should be.
So you get caught up in the whole shareholder mentality, and it's speculation. Yes, it is, and a fundamental rule of fiduciary law is that pensions should not be speculating.
Just because they shouldn't be making speculative investments using stock doesn't mean they shouldn't be making equity investments. We just need to use a different form of equity.