I do not know why chips from various casinos would be found in the same place, embedded in concrete but I can tell you something that I have heard.

When casinos retire chips, or at least at that time, they had to certify that the chips were permenently disposed of and or destroyed. One of the methods of destruction was to mix them into concrete or asphalt and pave parking lots or pour them into sidewalks, walls etc. I didn't notice but was there mention that all these chips came from the same source of concrete? I have heard that some people have found chunks of concrete that had been hauled away from these demolition sights and have chipped out the chips. (no pun intended). It was also mentioned that it might be possible that large quantities of chips may have been dumped in Lake Meade or burried in the desert... Who knows what might have happened to all the chips that have been retired over time.

Debris from demolitions or remodels, that contain chips, may be at the bottom of Lake Meade, Burried in the desert, used as fill on other construction sites... For example, when MIRAGE, tore down The Dunes to build Bellagio thay may have taken some of the debris and used it as fill to build up the landscape around the Bellagio. Some of this debris may be burried in the contours of all the Golf courses around Las Vegas. Anyways I am starting to wander here. There may be dumps, landscapes and other new construction with alot of debris out there that are full of old chips tokens etc.