Matt Levine writes a bit about this - the Elon Musk Mars Conglomerate. And really if you're investing into e.g. SpaceX you're not investing into SpaceX you're investing into the Elon Musk Mars Conglomerate. And most people seem to want that.
Tesla's the odd one out: it's public but it's still in there, although Musk would probably prefer it to be private too.
> Elon Musk Mars Conglomerate
That’s SpaceX’s version of Tesla’s self driving car pipe dream
Edit - I use self-driving car and Autopilot interchangeably
Tesla is the free cashflow play that is probably the most important for mars as there is no distilled fermented dinosaur juice on mars, but considerably more by ratio of lithium / oil than the Earth. Our flintstone fire mobiles won’t work so well there, and battery / solar will be important there for everything, including mobility and armies of slave robots.