Earlier quoted context omitted.
As others pointed out, when you buy/sell futures, you risk counterparty risk. But it's not that bad because that exchange is really good and people entering the market pay margin calls every day. There is also a financing cost (ie. the cash you use to pay for your margins, transaction costs, and to buy the bitcoins will not pay you any interests over the time of the position). But again, the effect is not very large.…
The bitcoin exchanges have plenty of liquidity, and executions happen in less than one second. The real risk for arbitrage is with the crypto exchange itself: these can go bust, or disappear overnight. You wanna keep $1M sitting on one of these exchanges?
I'm not sure a sensible institution would even consider BTC held at exchanges to be real BTC, just a kind of IOU. There may even be regulations preventing them from doing so. So they will probably want to hold the BTC themselves. Or with a clearer, which is likely to think about these things even more conservatively. And 20 minutes (the last figure I heard for how long it takes to execute a bitcoin transaction) is an eternity to an industry that is starting to think in nanoseconds.