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.

1. There's a limit to how large we can scale computers at any given tech level. What you're talking about is basically what a supercomputer is (they have many CPUs rather than one huge one), but there's still a limit to what's practical with them. 2. What do you mean by "evolve intelligence"? Run evolutionary algorithms on random bits of code? How do you evaluate the results? Before you can use search algorithms you have to be able to define the target, which is most of the problem in this case, plus search is likely to be impractically slow in something as big as "the space of all programs".