Why can't we just make a CPU as large as a dump truck [...?]

Lots of reasons, some of which Vaniver and ShardPhoenix have already given, but one of the big ones is that CPUs dissipate a truly enormous amount of heat for their size. Your average laptop I7 consumes about thirty watts, essentially all of which goes to heat one way or another, and it's about a centimeter square (the chip you see on the motherboard is bigger, but a lot of that is connections and housing). Let's call that about the size of a penny. That's an overestimate, but as we'll see, it won't matter much.

Now, a quick Google tells me that a dump truck can hold about 20 cubic meters (=20000 liters), and that a liter holds about 2000 closely packed pennies. So if we assume something with around the same packing and thermal efficiency, our dump truck-sized CPU will be putting out about 30 * 2000 * 20000 = 1.2 gigawatts of heat, or a bit more than the combined peak output of the two nuclear reactors powering a Nimitz-class aircraft carrier.

This poses certain design issues.