Probably the cost associated with storing, counting, insuring, securing, depositing the money. I get the privacy aspect but then why not get a prepaid credit card or something like that?
Well, then we're SOL and won't be able to charge our cars (fully) anonymously.
Converting >$10k from card to cash on the other hand often requires an ID
Canada requires retaining a record of the ID used to buy the prepaid credit cards, too.
https://fintrac-canafe.canada.ca/guidance-directives/client-...