Why can i see floaters in front of walls or sky?

Entoptics: You are probably seeing your circulating blood cells. This is normal and in the proper background you can see the cells swirl around in the blood vessel and disappear when it enters larger vessels. We call this entoptics and it can be a test for the quality of your central vision.

Because that's when: They are easiest to see. They are always present but you cannot see them unless you get the clutter of the world out of the way.