[jrockway]

One possibility is the amount of money to be gained. According to http://www.quora.com/What-is-Dropboxs-revenue, "revenue is on track to hit $240 million in 2011". Google, on the other hand, made $37,905 million in 2011. Microsoft made $69,943. Apple requires too much effort for me to get the numbers, but it's more money than either Microsoft or Google.

So my theory is, "why bother?" If one of those companies needed a few more million, there are easier ways of making the money.