This is the only answer I've heard so far that makes sense: cash-rich companies (and investors) have money, and want to spend it.

"It turns out, there's a lot of money in the world. The money is bored. Money wants to be spent! Money is intended to be gambled. These are big numbers, but these are big companies." - Jason Calacanis, http://youtu.be/SY62KuoxWJ0 [Video]