"For other industries, though, a dollar of savings is worth a lot more than itself. For pharmaceutical companies, a dollar in savings is worth $1.50. For software firms, it’s even higher: more than $2. This means that investors are behaving as if they trust the executives in these industries, like Larry Page of Alphabet, to be smarter about using that money than the investors themselves could be. ...
"Why? The answer, perhaps, is that both the executives and the investors in these industries believe that something big is coming, but — this is crucial — they’re not sure what it will be. Through the 20th century, as we shifted from a horse-and-sun-powered agrarian economy to an electricity-and-motor-powered industrial economy to a silicon-based information economy, it was clear that every company had to invest in the new thing that was coming. These were big, expensive investments in buildings and machinery and computer technology. Today, though, value is created far more through new ideas and new ways of interaction. Ideas appear and spread much more quickly, and their worth is much harder to estimate."
Source: http://www.nytimes.com/2016/01/24/magazine/why-are-corporati...
The implication is that big tech companies see some very disruptive trends coming down the pipeline, but they're not sure which specific idea or ideas should take most of the investment, and they're hedging their bets by hoarding cash for now. Maybe when the AI/automation revolution kicks into high gear, we'll finally see what they spend all this money on.