Because people who have successfully acquired massive amounts of wealth and power know full well that opening up access to space will yield more material and energy wealth than any previous leap human civilization has ever made.

Small thinkers are satisfied keeping their imagination on Earth alone, but small thinkers don't lead massive leaps of prosperity.

Musk: Inspired by having a backup for humanity on Mars, and an aspirational goal to motivate everybody

Bezos: Space will be industrialized and earth will remain some kind of national park

Richard Branson: He doesn't give a shit other then to self aggrandize himself further as far as I can tell.

Space is going to be industrialized, sooner or later. Since it takes massive amount of capital to do so, you can pretty much limit this kind of thing to wealthy nations or multi-billionaires such as Musk, Bezos and Branson.