I mine on Mingspeed pool and when my payouts dropped today I checked their pool stats here https://pool.miningspeed.com/workers.  The top miner had far and away more hashing power than anyone else.  I clicked on their address and that is where the image came from.

To give them an unsolicited plug, their stats pages are quite nice and I am pleased with the pool.  It is a nice middle ground -- small enough that you are not lost yet big enough that they find a block several times an hour.

As to why mine vs buy, I can only speculate that they think they can mine it more cost effectively (cheaply?) than they can buy it.  Or maybe they worried that they could not buy that much without driving up the price.