You're right, the claims would eat into their costs if doing this once. But with a claim [1] they could mine 24/7/365 and even bring cheap labor along assuming they know what they are doing. Another advantage of working a claim is that one does not have to rush or cut corners further increasing risk. Some of those have partially covered multi-level stopes. One wrong step can send someone several hundred feet down jagged rocks.

[1] - https://www.blm.gov/programs/energy-and-minerals/mining-and-...