That's from the ancient banking system designed in the 1940s that is still lurching around. The protection on a check isn't cryptographic - it relies on the issuing bank to confirm the authenticity by examining heuristics on the paper check like paper type, ink and font used, check number, issuing address and the person's signature. It used to be that you would deposit a check at your bank and then it would be mailed to the issuing bank to be cleared for transfer. You wouldn't just deposit it and get your money instantly, it would take up to 10 days.

Honestly it is bewildering to me that they haven't changed the system to issue cryptographically secure deposit-only numbers and unique withdrawal numbers that at least verify the authenticity of the check itself.