[nostrademons]

Not sure if you were at Google in 2009 when this question came up in the immediate aftermath of the financial crisis, but the answer is that opportunities are discontinuous. Economics in the real world doesn't work like "spend $x, make $y = $2x" in any sort of high-tech industry. Rather, it's "wait 10 years for the appropriate technology to be developed, spend whatever it takes to acquire it, then you have the opportunity to spend $x for the chance to make $100x". The gating factor is the development of the technology itself, which is unpredictable and depends upon certain intuitive leaps of faith by many, many researchers, sometimes working in concert and sometimes just exchanging ideas by chance.

Holding large cash reserves gives these big companies a chance to pounce when an opportunity does present itself. That's also why prices for genuine innovation tend to be kinda insane. It's a market with one seller and max 3-4 buyers, so not exactly competitive.