This is called precession. It is just like the wobbling of a spinning top. Earth did not originally have a lot of free oxygen in the atmosphere. The oxygen was mostly combined with other elements to make carbon dioxide, water, or in rocks. But as early life evolved, especially plants, well they give off oxygen.
Objects hitting our atmosphere usually burn up because they are hitting it so fast.
Usually they are moving at hundreds or thousands of miles per second. Here's a comparison. If you jump into a swimming pool from the high board and hit the water really hard, it stings and you make a big splash. But if you step into the water slowly, it feels fine and you make only a little ripple. It could cause a big explosion.