Because many of these companies have millions, sometimes billions of dollars (or worse, other fiat) that is deprecating at an average of 3%, if you think that those numbers are real, which probably now, so in practice it's more like 5-6% yearly debasement. Companies cannot buy the SP500, and I think they also cannot buy an ETF (which is the vehicle they would use to store gold, since they are not going to bother to hold their own gold). And so that is how Saylor arrived at the conclusion: BTC is the ultimate hedge against fiat debasement. You are legally able to hold as much as you want, and it's easy to setup compared to storing your own gold bullions. It also gives you a new following of millions of people around the world that suddenly care about your company, free publicity. It is a winning strategy as long as you can deal with the volatility.