Today most mining for ores involve digging beneath Earth's surface. Why is this necessary?

Because most metal ores are neither gaseous nor liquid?

Honestly, I'm not getting why this is even a question. Where would you expect them to be, lying around in convenient piles on the surface? If so, what conceivable process would produce such a thing?

Even if it did happen, you can bet that those would have long ago been found and picked up. If you could go somewhere and just walk around and pick up gold nuggets, someone else would have already done it.