[PapaBear3625]

The Martian atmosphere is 96% CO2, so once you have plants growing, you will have oxygen.

[Carl Vehse]

Mars doesn’t have enough gravity to retain oxygen or water vapor in the atmosphere.

[Windflier]

Mars has no molten core, thus no magnetic field to shield the planet from our sun’s solar wind. Because of that, it can never retain an earth-like atmosphere.

Perhaps one day humans will figure out how to restart the planet’s internal dynamo, which would create a magnetic field. It’s the only way to build a breathable atmosphere on Mars.