The thing is that Mars settling is not an idea Musk thought up. It's something that space pioneers have been pursuing since the dawn of the space age or even slightly before. Tsiolkovsky and the other Russian (and later Soviet) Cosmists. Robert Goddard and Wernher Von Braun. And it was kind of the understood backdrop narrative to NASA's foundation and Apollo, why it seemed to capture everyone's imagination. Yeah, scifi. But scifi is not supposed to be a synonym for the impossible or fantastic (ala Star Wars). Jules Verne (for instance) is scifi but powerfully predictive in what technologies ended up existing (nuclear submarines, space travel in capsules, etc). People EXPECTED space settlement after people landing on the Moon and were excited about it. I think the Cold War eventually kind of destroyed that optimism for a while just due to dread and dystopia becomingfully mainstream in media and scifi.
NASA never gave up human missions to Mars as a goal (and not just supporting the search for life, but as a goal in and of itself, with permanent presence as the goal). Apollo was wound down in favor of Shuttle, which was to lay the groundwork for Mars by lowering launch costs and establishing Space Station Freedom (which became ISS) as a platform (in part) for building and launching missions to Mars. The Soviets were pursuing a similar path. After losing the Moon race, they chose to refine long-duration human spaceflight instead, developing the Salyut space stations built with logistical support from many launches (a first for spaceflight, and considered a prerequisite for Mars). Salyut was considered a kind of precursor to a Mars transfer vehicle. Salyut expanded to the Mir program, whose spares and follow-ons ended up contributing to the critical propulsion elements of ISS. But like NASA, the goal was still eventually Mars and space settlement. (And around this time in the US, there were also space settling enthusiasts focused on just scaling up space stations to be large, permanent settlements... a kind of rival for the Mars sect).
In the 1980s and 90s, there were a group of Mars enthusiasts in industry and NASA that started pushing Mars settlement hardcore after a few decades of post-Apollo disillusionment. They saw ISS and space stations in general as a kind of distraction, a needless cost and time tax for missions to Mars. They were (are) led by Robert Zubrin, who developed an architecture for Mars exploration called Mars Direct that could do cargo or crew launches to Mars in just one launch, with rendezvous with the different elements happening on the surface of Mars, not in orbit. They also did some important work on in situ resource utilization (ISRU), i.e. the idea of producing the vast majority (or all) of the fuel, oxidizer, and even consumables like water and air from resources on Mars (initially relying primarily on Mars' CO2 atmosphere, but later incorporating surface ice as knowledge about the extent of water on Mars expanded), convincing NASA to hesitantly baseline ISRU for future missions (previously had been Apollo-like all-up missions bringing all supplies from Earth). They were more vocal about permanent settlement on Mars as the end goal than NASA. Robert Zubrin and his group established The Mars Society in 1997, with permanent settling of Mars as a primary goal. Today, The Mars Society has Mars analogue research stations around the world where volunteers dress in mock spacesuits and live in mock space habitats (naturally, modeled after the ones from Mars Direct) and conduct mock Mars missions, including studying rock samples from the surrounding area (usually some kind of remote desert). This has been going on for decades, now.
Elon Musk, shortly after selling Paypal and getting rich, became kind of converted into this Mars settlement vision. He donated a bunch of money to the Mars Society (they named a telescope at one of the Mars analogue research stations after him), and they convinced him to try to send a greenhouse to Mars to kickstart the public's imagination on Mars, with the goal of increasing NASA's budget and focusing them on the humans to Mars goal. Long story short, he tried buying ICBMs from Russia on the cheap, was rebuffed, and so ended up starting SpaceX (and the ambition has grown tremendously from that point on).
So I'm not surprised not everyone is on the Mars settlement train. You've got to catch the Mars bug. It's not for everyone. But it's much bigger than Musk alone. It's an old dream going back over a century. Here's the Mars Anthem, with words written by Robert Zubrin, describing in grandiose terms the goals of the Mars Society:
https://www.youtube.com/watch?v=cj8-NcyULps