The entire market for every metal they could mine is larger than any single market. Gold, Platinum, Rhenium, Palladium, Iridium, Rhodium, Indium, Osmium, etc. Also, it can be a question of timing. If it suits them to sit on the equivalent of 10 years of global production of, say, Platinum while selling off, say, 50% of global production a year then that's still an enormously profitable venture for them.