It's not just risk. It's complexity. Groupon is a single product. No single product is going to put human colonies on another planet. No single company, or even single government, seems likely to do that. What, precisely, is Sequoia or Kleiner supposed to throw a giant check at? The rocket maker? The rocket fuel maker? The habitat designer? The logistics and supply companies? The cryogenics company? Lots and lots of moving parts and interdependencies involved, and no apparently clear winner to take the lion's share of the financial upside.
Everywhere. Look at the numbers - Groupon has only taken a total of 1B in investments. SpaceX recently took 1B alone from Google in a single investment - and on top of the public billions spent on NASA, and private billions spent on SpaceX, Boeing, Lockheed - and the numerous other academic research projects.
The difference, I'm sure, is publicity. I'm sure the Groupon IPO got a ton more press than the curiosity landing. You simply don't hear about how the billions are being spent on those project because they don't have relatively massive returns in 5-8 years like a social coupon company would, and large money is spent on relatively small, uninteresting steps.
Likewise it isn't wise for a private fund to aggressively invest in these spaces. What would be an individual firm's financial return on developing the internet? A lot less than the financial return on Google.
The other difference is capturing the surplus. Fixing the economy would be worth trillions, but how do you get your piece of it? It's much easier to capture the surplus if you can insert yourself into the payment process for an already existing product.
That's why IMHO only governments can do those kinds of things right now. Only governments can make high-risk-high-payoff extremely broad and long-duration investments and then capture the upside through overall economy-wide growth (via taxation and other means). Nobody else can both afford the risk and capture some of the upside.
It's possible that in the future someone will invent financial vehicles to make it possible to make those kinds of investments in the private sector, but it hasn't happened yet.
Edit: in a sense there is already something, namely government bonds, ETFs, and long-term positions in currency markets. But those are too coarse-grained. What we need is some way for private investors to make private investments in things that are all of: high risk, high payoff, and broad payoff.
So where's the investment capital for them then? They're all chronically underfunded.
The difference, I think, is risk. Any one of those would be worth trillions if it worked, but the risk of any approach failing is very high.