What if they mined when it wasn't being used or whatever? Surely they've heard of Bitcoin and if they already have the hardware, why not? Relative electricity costs would probably be next to nothing with such an efficient computer. It seems stupid for an investment company not to maximize their own investments. Assuming that they aren't doing it because they are too good for a nerdy currency is foolish.
Because it's not worth it to them.
They earned 5.6 BILLION this past quarter. The entire bitcoin market cap is under 100 million. Yes, those numbers are big for us but they don't even bat an eye when looking at it from their view.