Why not a mining operation in the Antarctic? Or the Sahara? Or in the Mariana Trench?

As all these options are cheaper and easier than asteroid mining, and as none of these bright-eyed ‘entrepreneurs’ are undertaking these tasks, I can only assume the things being ‘mined’ are gullible investors. (Not that I’m blaming them… what’s easier? Finding an asteroid with a precious metal, mining it and getting it back to earth at less the cost of mining it here? Or, finding a gullible venture capitalist who will cover your salary for a few years? Probable the latter…)