But you can only have so much cash in your pocket. The bitcoin private key is 32 bytes always, regardless of the amount.
For example here is the famous 37 BTC ($500,000+) donation to Andreas Antonopoulos yesterday, this couldn't be done with cash, at least not as seamlessly, inexpensively and quickly:
https://blockchain.info/tx/e68ebe16e6aaa08e3f9d271ba78aaeb4c...