Remember: Today's air filters live above a velocity drop box.
Dirty air enters through one hole or another then it's immediately in a large box (below the filter element) where the air's velocity drops.
Large (as in heavy) dirt precipitates out and only the finer stuff reaches the element. That's why they don't "look dirty".
The funny thing with my car was that soon after I bought it the filter was 30,000 miles old. I'm happy to change them then.
I lifted the element out and gobs of sand fell out. There was a lot of it to be sure.
I had no thought of tapping the sand out and possibly reusing the element thought I'm sure the "100,000 mile guys" would scold me for wastefulness.