Why does earth have oxygen but Mars and Venus do not?

Answer 1

Their are many different reasons for this, it depends on when the planet is formed how close it is to the sun, so our planet just formed at the right place at the right time.

Answer 2

Earth has oxygen-rich atmosphere primarily due to the process of photosynthesis performed by plants and other organisms. Mars and Venus, on the other hand, do not have significant amounts of oxygen in their atmosphere due to different geological and atmospheric compositions that do not support the production or retention of oxygen in the same way as Earth.