In Canada, it's required by Know Your Customer (KYC) legislation. It's anti-money laundering and anti-crime legislation.

There aren't many legitimate reasons why people need like $10K in literal cash. Usually, that much money should be sent with a money order (bank draft). That is, unless you're running something like a major retail store, in which case you'll have arrangements to get lots of small bills and coins regularly.

Banks also just don't have that much physical cash on hand. They don't want to risk worry about them running out of cash since that can trigger a run on the bank. They can get you as much cash as you need for any reasonable purpose, with notice, but not every branch is going to have $100K+ sitting around already sorted and ready to go so every rando in town can get tonnes of cash out on the same day. If you're running a comic con and need $50K in coins and $5s for making change, the can get that for you no problem next week, but ain't nobody going to help you today.

We also don't want to make it easy for criminals, like human traffickers, to make large cash withdrawals and deposits.