A lot of people are mentioning the Earth's atmosphere here, and it just isn't that big of a factor for sizable impactors over history. Yes, it keeps dust-sized particles from making a constant stream of micro-craters, and stops the fist-sized rocks from making small craters, but even Venus with 90 times Earth's atmosphere doesn't stop any craters bigger than about 2 km from being formed.

Okay, we wouldn't be seeing many craters bigger than the atmospheric cutoff being formed today because the influx of such objects is currently very small, but if we could have had the protection of our atmosphere while somehow turning off erosion/volcanism/tectonism for the past 4.5 billion years, then we would look (from a distance) as cratered as the Moon does--only when looking at small scales would we notice the difference.