Mercury: tidal lock to Sun, heavy solar radiation and solar flares up close, complicated orbital movement, low gravity, basically no atmosphere. May be habitable underground in particular belts with reasonable temperatures.
Venus: a runaway greenhouse effect, metal-melting temperatures and colossal pressure on the surface, colossal never-clearing clouds, very strong winds, sulfur and chlorine abound in the atmosphere. Could be habitable using artificial islands floating in the upper atmosphere where the temperature and pressure are comfortable. At least the gravity is close to that on Earth.
Jupiter is not habitable by itself, just due to the very strong gravity. Various moons of Jupiter are all pretty cold (though some have liquid water under the surface), and at his distance from Sun, the amount of sunlight reaching them is rather small, even when reflected by the giant disc of Jupiter in the sky. It's cold, solar power in impractical, uranium and thorium are not apparently abundant.
Saturn: see Jupiter, only even farther and colder.
Neptune: see Jupiter, but even without serious moons.
Pluto / Charon binary: not even a planet. At this distance, Sun is but another star in the sky.
Compared to these, Mars, with its conditions that resemble Earths's polar regions over large swaths of the surface, acceptable distance from Sun, serious amounts of water (as ice), acceptable gravity, acceptable amounts of sunlight, and relative closeness to Earth looks not that bad at all, and basically the only practical candidate for an autonomous, self-sufficient world.
People would have bone density problems much sooner in life, as an example.
Maybe we can try and terraform it in the meantime.
There is a decent chance that our present biological basis is not congruent with the future of humanity. Mind-extension into synthetic cognitive structures is not as far fetched as it seems- we already extend our consciousness using machines, albeit with a very poor interface.
It is foreseeable that we will come to understand the nature of sentience - after all, many simpler organisms exhibit sentience- and beyond that it seems to be a scaling problem, which are inherently tractable.
If we can improve the interface between biological computation and synthetic computation, it will likely become possible to extend sentience across that divide, barring some kind of spiritual phenomenon or “magic “.
If we don’t shoot ourselves in the foot too badly, it is foreseeable that we may make very substantial progress in this direction over the next century. Establishing a toehold on mars during that century will be a huge strategic advantage.
The problem with “people these days” is that almost no one thinks a multi decade project is worth doing. If everyone waits for the future to “arrive “, the future you end up with is just driven by quarterlies.
The most important accomplishments will be remembered for centuries. If you aren’t working on one in some capacity, what are you even doing with your life?
But I think you underlined the issue of timelines. Yes we can technically go to Mars in only a few years, but people living there permanently is going to be beyond my lifetime. Unless some people are willing to drastically sacrifice their health.
Mars itself is very inhospitable to humans.