Why would they sell them if they could just use them to mine the coins.
Because coins are not based on some real hard valuable thing, I cant easily go to kroger and buy milk with it, the price changes, it could crash at anytime. Why throw millions into it, when you could physically hold millions from people who are willing to throw money into it?
They sell them for their safety, for their living.
Also, not sure how many investors accept bitcoins to pay off hundreds of thousands in debt.
Also, say I am BFL and I make a few of the $29k machines. Mine all blocks for weeks. People will get pissed, leave BTC, you left with a bunch of BTC and no one who wants to play with you. All money into toilet.
You can easily exchange BTC for whatever currency you wanted as soon as you mine it. And you don't need to use BTC to buy milk or pay off debt, you can simply use an exchange to trade it for the required currency.