hyperalloy. The hyperalloy is said to be "a durable metallic alloy of unknown composition", but lets face it metal is a rare element on earth. Sure the core is 1220km ball of molten iron surrounded by a 2180km ball of molten nickel iron alloy. If you could drill to the core and extract all that metal it would sure provide for a lot, I mean a shitload of a lot of T-800 units. But that's a very big if, not to mention that said core provides the magnetic field for the planet. Take it away to build a shitload of T-800 and you'd have a whole new set of problems to solve, and I mean whole set of planet wide problems.
Coltan is an element used to build the higher heat resistant terminator units (vs the titanium based T-600). Using coltan, practical as it may seem for heat issues, is adding scarcity to scarcity. Silicon is also known to handle heat well. A lot of advanced ceramics are very good at handling heat (and can be made superconductive too!). Silicon is nearly and order of magnitude more common than iron on Earth's crust. If you're a super advanced computer from the future, doesn't it make sense to use your super advanced technology to build super advanced machines from the more common elements and not the scarcest?
There's a reason life built itself around carbon and not another element like iron. Carbon is extremely common! It also gives room for a huge diversity in structures and molecules: the soft and brittle insides of your pencils, the diamonds in industrial drills, nanotubes, DNA, the exoskeleton of insects, etc. Maybe the truly advanced robot from the future is a carbon based, ceramic enhanced, nanotube fuel cell powered, quantum computer driven machine more akin to David or Bishop than the T-800.
Sure, maybe a coltan built robot will be an order of magnitude more durable than an "organic" one, but given the abundance of materials there might be two or three orders of magnitude more organic bots. Numbers do count and ammo runs out sooner or later.