Why are computers not cubes?
Yes, I can think of at least two.

The natural construction of a computer is to put as much stuff as possible on the motherboard - a flat PCB. You can then have expansion cards sideways (PC-style) plus peripheral disks inside on cables. The "flattish" form factor lends itself to laptops, 1U rackmount systems, and set-top-boxes.

The other reason is heat dissipation. For this you tend to want a high surface-area-to-volume ratio and good laminar airflow through the system. A cube is the opposite of that unless you can make one face a fan. Again, if you look in 1/2/4U cases a surprising amount of space is full of ducting to make this work.

Steve Jobs agreed with you that cubes are cool, which is why he tried to make the PowerMac G4 Cube. It was generally considered to be a cool-looking failure due to heat and expandability problems.