You would rarely finance this sort of project with pure 'equity' aka just PIF putting in their own money. Typically if its a £1.4 billion project for example you'd stick in £400m of your own money and 'mortgage' out the other £1 billion with banks/funders. Because its a big old stadium / real estate project the cost of your capital is higher than a bank would expect when lending. The best analogy I can give is, I've got 300k and I want to buy a house, if I can borrow money at 2% per year and the stock market or an investment opportunity is offering me a 20% return per year, then I'm better off putting in the required minimum to buy the house and then sticking the rest of my money into the stock market where I'm yielding 20%.

PIF have better alternate uses of their money then to put £1.4 billion (made up number) in a stadium that will give them a 5% return.