These laws are a balancing act between heritage conservation and civil rights like allowing people to enjoy metal detecting as a hobby. Without these laws and effective enforcement of them, looters show up with bulldozers, metal detectors, and work crews in the middle of the night to dig up anything they can sell to what's now a billion dollar global market. I've spent a lot of time as an archaeologist looking at the aftermath and doing public education to stop it.

Antiquities laws are generally designed to make antiquities "visible" to the state while also giving authorities teeth if they need to go after these sorts of people. To do that, there's a whole host of laws and conventions that require reporting virtually everything and align the financial incentives/penalties to ensure valuations on any antiquities discovered are as reasonably accurate as possible. It makes things a little inconvenient for the hobbyists, but I personally find that a reasonable tradeoff given the damage that otherwise occurs.