Mars is a meme planet with no real resources or reason to settle aside from the reason that it looks cool. The planet's core is essentially dead, meaning there's no magnetic field to protect the surface from deadly space radiation. As a result, all colonial buildings would have to be built underground and pressurised, just like a space habitat. Not too bad, right? Well, Mars' atmosphere is thin enough that, for human purposes, it's a vaccuum, but for attempts to land on the planet, it's the worst kind of atmosphere. Solar wind blows away any significant atmosphere but there's just enough there to be awkward - spacecraft landing on Mars will burn up in the atmosphere if they are built with no consideration for atmosphere but the atmosphere is also too thin to be used for direct aerobraking (such as landings on Earth or Titan) - requiring a joint mechanism for landing that incorporates rocket engines and parachutes, increasing complexity and cost for any mission landing on Mars. This is less space useful for "dry" mass (people, food, water, supplies, anything that isn't the spacecraft structure + fuel essentially).
What else does Mars have? Well, there's the one-third Earth gravity that could cause the potential for muscle atrophy and reduced bone density (among other potential health risks we can't estimate yet), particularly for those growing up in such an environment. People growing up on Mars and living their entire lives there would have to perform rigourous training regimens to ever be able to go back to Earth-normal gravity, as otherwise their bodies would weigh three times as much as they would be used to. What about the drastic temperature variation that Mars recieves every day-night cycle that would quickly wear down and destroy exposed components such as communications equipment, I mean that's pretty drastic. This is all before considering that Mars has no particular natural resources that make it suited for habitation. Where Ceres, Deimos, Ganymede, Callisto and Europa have multiple-Earth's-worth of water ice, Titan has ample hydrocarbon lakes and the biggest heat sink in the solar system or Mercury has sunlight, Mars has nothing. Want minerals? The asteroid belt has you covered by an order of magnitude more, while also being much cheaper and less costly in regards to Delta-V to obtain. Even Mars' moons Deimos and Phobos are incredibly useful staging areas, as their essentially null gravity makes landing and extraction of ice easy, but their gravel-esque composition makes them easy to hollow out for cheap radiation shielding. But even then, if you can protect against radiation on Mars, you can do it a million times cheaper in space, while also being able to control the gravity of your habitat because of based centrifugal force, meaning you can easily live in 1G gravity your entire life.
This is all before considering that terraforming is a project that would require at least a civilisation at level two on the Kardashev scale (complete control of all resources of a given star) - where we only rank at barely 0.7 (not even capable of fully harnessing our own planet). Good luck trying to melt a literal planet's worth of mass to restart the core for that magnetic field, and another planet's worth to increase the gravity to Earth's. Not to mention the question of where you're going to get all of that water from to fill the oceans and the air to breathe, because all that mass has to come from somewhere too.
It's probably just easier to build a shit ton of space stations in a Dyson Sphere orbiting the sun tbh fam. Planets are more trouble than they're worth.
>the virgin mars
>the chad o'neill cylinder