Why would they? Yes, someone with millions to spend could be working out the kinks and fine tuning a 12nm chip in secrecy, but that would not only be more expensive and tedious than stepping down in size using well established and much less expensive methods of production it would also leap frog over billions or potentially trillions of dollars in profit over the next few decades.
I am awaiting the day that samsung, intel, amd, et al decide to dip their toes in the crypto mining game. Intel and AMD both have the experience and the facilities to produce 12 and 14nm tech at much lower costs than the R&D needed by companies who have been around for a fraction of the time. If we ever do see a Bitcoin brothers type of new hardware come online it will be because these companies decided to test the waters.