They are asking because the US Dept of Treasury/FinCEN requires them to do reporting on large cash transactions, typically called a CTR, or currency transaction report. It is part of the anti-money-laundering statute. All financial institutions (banks, casinos, etc.) are required to follow it.
A CTR is required at 10k or above per day, but in order to make sure people aren't structuring to avoid the reporting thresholds, most casinos (and banks) collect info at levels of 2-5k instead for tracking purposes (i.e. it may just be internal, not reported as a CTR, until you hit the 10k number in a day).
Typically if you don't provide it, they will instead log a SAR, or Suspicious Activity Report, on you instead. This probably makes it more likely that DoT will secretly investigate you.
If you're not doing anything wrong, then none of this will matter most likely, other than the hassle of arguing about it every time you go to cash out. Or they may (as you note) also refuse to do business with you in the future, which many of us would consider to be quite painful.
You can search for "CTR" in this forum to find many previous posts about this topic. Possibly this link might work as a shortcut.
https://forumserver.twoplustwo.com/s...rchid=66770250