Over seasons the dirt gets mixed with mainly rubber, but also oil, fuel, exhaust gasses. It also loses any organic material that may bind it together. Ever notice how farm field drives turn to powder when it hasn't rained?

Over a weekend it's mainly(IMO) because it's not feasible to bust the track up to get enough water in it then roll it back in again for the next night. Most promoters would rather have it black top to bottom and smooth for the second night than have it crater up.

There are exceptions right? Everyone's been to a track where they're busting it up before the people are even out of the grandstands. Just know when you see them do that that someone's probably going to be on that track a good part of the night.