Why do future interstellar immigrants choose Mars first?

It won't be Venus, because the environment on the surface of Venus is so bad that it is not suitable for human habitation at all. The surface temperature of 480 degrees Celsius, huge atmospheric pressure, lightning that happens all the time, extremely high concentration of carbon dioxide, and concentrated sulfuric acid fog floating in the atmosphere can all kill people. Mars is possible, but there is no oxygen in the atmosphere of Mars, and human beings can only rely on their own oxygen.