Disinflation & deflation, capital flows and political friction preventing effective (any?) fiscal policy have produced an environment where private capital set on a given rate of return on investment is chasing increasingly risky organizations [1][2].
Large firms have been sitting on enormous sums of cash; e.g., why is it that the most capitalized company on earth isn't investing aggressively. In lieu of investment, many of these firms have been focused on engineering stock buybacks.
So, the thesis: why are so many firms pouring money into startups with increasingly questionable fundamentals? Because hands previously gripping bundles of capital have (nominally) more capital than they did with decreasing options for productive investment and downward pressure on returns.
1. http://www.economist.com/blogs/freeexchange/2015/04/puzzles
2. http://krugman.blogs.nytimes.com/?s=low+inflation+return+inv...
P.S. I recognize that I'm probably a bit left field for this group as I'm not a libertarian, I support strong regulation and I question the marginal value of lots of Valley products.