Rocks & Dirt Inside Under-body Panels

Just an FYI. I changed oil last weekend and noticed a fair amount of small gravel pieces, dirt and other debris on the inside on the top of the engine under-body panel after removal. This prompted me to check and remove the other 4 panels and I was surprised to find a fair amount of the same debris as well. I rarely drive on grave roads (less than 1%) and if I do it's at slow speeds. Can't quite understand how this stuff gets in but it does. The removal level of difficulty is low but the time it takes is longer than I'd like (rather be doing a lot of other things besides crawling under a car on cardboard unscrewing panel fasteners!). I scrubbed them with Simple Green and hose them off. That's pretty much it.