To better understand, look at your list of engineers/entrepeneurs first:

-Paul Allen: Acquired QDOS for Microsoft, worked with folks who saw potential in software instead of hardware for desktops.

-John Carmack: Cofounded id software, code is especially notable for being flexibly licensed, open for modification, and for being built with the next-generation of computers in mind. Consider that Quake required an FPU when they weren't always common, and that the first and third games in that series ran the game logic in a virtual machine (a VM, for an FPS, in the 90s, on consumer hardware).

-Richard Branson: Built up empire in music industry by taking risks, solidified by expanding into transportation by attempting (if I'm not mistaken) something like Southwest for Europe, and expanded into telecom as first network not needing its own infrastructure.

-Jeff Bezos: Built up Amazon, and insisted on long-term solutions. Because of this, significant chunks of the infrastructure we all use today run through projects like AWS.

-Mark Shuttleworth: Founded Ubuntu Foundation and laid the groundwork for Linux on the Desktop (a running joke now, but still a valid goal). Later work was making the decision to switch from X to Wayland, which one day may pay off.

A common trend among all of these folks is their ability to forgo the "easy" solution in favor of a longer-term, harder, but ultimately more profitable one. Their decision to look at the long term is what has made them successful, and in most of their cases their contemporaries likely saw them as being either silly or nuts (or both).

So, why aren't they investing in {nanotech,robotics,cold fusion}?

Nanotech is a great buzzword. My khakis have nanotech. I think the toothpaste I used this morning has nanotech. When you actually talk with folks in the field, though, it doesn't really serve a great purpose other than securing NSF grants. There is fantastic basic research in materials science and chip fabrication going on, but the idea of doing something as outright amazing as a molecular assembler is pretty far from where we are now--and anything else is basic engineering that frankly doesn't do much for the species as a whole.

Robotics is another great buzzword. Here's the issue in a nutshell, though: any robot you show me is going to be less versatile than a human (of even minimal intelligence and skill), and will take more effort to manufacture. To put it differently, the human body is the most complex mechanism on Earth, and the only one that can be produced by entirely unskilled labor. Robotics' main purpose these days seems to be either cleaning or helping one group of humans oppress another--GM tried to use them in mass number in the 80s and damn near wrecked the company. Why do we need robots? Why does the species (not just some first-world folks) benefit from them? Why not just use people?

Cold fusion has not been shown to be possible, and (for the worse) the entire topic is ridiculed and somewhat of a pariah in the only circles that can really do useful work on it. Some folks still press on (godspeed Focardi!), but the timescale of actual progress is not looking good. This is really sad, because cheap energy actually benefits the species as a whole, and would be unconditionally a good thing, for everyone (third- and first-world alike).

So, finally,let's talk about space.

We have a limited amount of resources on this planet--even setting aside arguments about how the resources are distributed, it is obvious that one day we will hit peak. It might takes decades, or centuries, but it will happen. With our current cultural emphasis on growth (and the attendant acceleration of consumption of resources), we are only getting into a worse position.

Sure, we can limit ourselves and attempt sustainability, but the burden of that (reducing consumption and procreation, etc.) is a very large one, and it seems that the only places with the wealth and technology to take the long view are the same ones with the populations most resistant to loss of creature comforts and centralized control (almost certainly necessary).

Long term, we can stabilize on Earth and go out with a whimper instead of a bang, or we can try to do better.

Space is better. Leaving to procure more resources is better. Distributing the human race so no cataclysmic event can wipe us all out is better. Exploring our universe (or even, initially, our solar system) is better.

Any other course of action, regardless of how humanitarian or efficient or resourceful, is picking out ahead of time the color on the coffin of the species.