They are voxels, they just have very specific placement rules. They take up a full block but can't be stacked. There's also the fact that the wires transmit in

They are voxels, they just have very specific placement rules. They take up a full block but can't be stacked. There's also the fact that the wires transmit information only 15 tiles. It's possible to build repeaters though, but those repeaters have some lag and take up space. The hardest part are the memory cells. A single bit, even in the optimal configuration, takes up 3x3x2 blocks.