EXTORTING MONEY FROM MEXICO

I really can't understand why Mexico doesn't agree to let Trump extort some $20 billion to build his wall.  After all the value of the American-owned factories, in Mexico, that they can confiscate, surely can cover the cost of the wall.  In any event, monetary loss if Trump imposes a 20% tax on Mexican imports, will be borne by those same American companies, since the tax will be upon their own goods, for the most part.