In France, when people deposit more than 10k€, the bank has to notify Tracfin, the service of the French Ministry of Finances that fights money laundering. And you have to be able to explain where the money is coming from.

Most of the time, people with large amounts of cash are workers (skilled construction workers...) that are able to get a significant part of their income in cash and do income tax avoidance. Or criminals (drug dealers...).

Having to deposit their hidden savings all-at-once is not good news for them. This is basically what happened when France switched to the Euro.