The thing that stops them is the lack of a reason to start them. Why would Goldman throw that much money at a new system, rather than using the existing one? Bitcoin has an existing user and development base; it's proven, whether it's at full scale or not. Goldman does not have a guarantee that their hypothetical coin will gain any acceptance outside their walls. And Goldman would have to ask themselves that same question - "If we can create a successful coin overnight, why can't BoA or Chase?" If Goldman can do it, their competitors can, so that $8 billion they apparently have sitting around with no better investments to make is better spent on getting an early hold on Bitcoin.
And, of course, this all assumes that big banks are forward thinking enough to see coins as worthwhile, and that they have the kind of developer power in a new and obscure area to outperform 5 years of FOSS development before anyone else beats them to it.
What stops Goldman Sachs from seeding a cryptocurrency tomorrow with a market cap that massively eclipses BTC? The big IBanks are sloshing billions and billions of dollars around all the time. There is this assumption that BitCoin is so big that it is the winner, but it is "so big" relative to a bunch of people that aren't in the world financial system today.