Question:

Why are all craters on Earth at exactly a 90 degree angle to the "flat" surface?

Not a single one has ever had any inclination or angle of trajectory prior to impact.

If we live on a "spinning ball in space", what are the odds that every single impact from "space rocks" strikes the Earth straight down, at a perfect 90 degree angle?

They are actually the fountains of the great deep.

Genesis 7:11 KJB
[11] In the six hundredth year of Noah's life, in the second month, the seventeenth day of the month, the same day were all the fountains of the great deep broken up, and the windows of heaven were opened.