Why isn't Mars habitable?

Mars has an atmosphere, but barely. Average surface pressure is 6.36 mb, with seasonal variation between 4.0 and 8.7 mb — a tiny fraction of sea-level pressure on Earth. Its composition is no use to a human lung either: 95.1% carbon dioxide, 2.59% nitrogen, and 1.94% argon, with oxygen not among the main components. Bottled air alone wouldn't be enough there; what's needed above all is pressure, so any activity requires a pressurized suit or a sealed habitat.

Martian temperatures range from about 20 °C at the best time of day and year down to about -153 °C, a range no Earth organism can survive unprotected. The added problem is water: the atmosphere is too thin for liquid water to persist on the surface for long, so it shows up as ice just beneath the ground in the polar regions and as brines that flow seasonally down some slopes and crater walls. With that pressure and without stable liquid water, the chemistry of life as we know it has little room to work, and scientists don't expect to find living organisms thriving on Mars today.

The distinction matters. Martian surface gravity is 3.73 m/s², 0.38 times Earth's, enough to walk and work in. There's accessible ice, carbon dioxide as a raw material, and sunlight. That's why crewed exploration plans don't talk about finding a habitable world, but about bringing habitability along: pressure, oxygen, water, and thermal protection manufactured and maintained by the mission itself.