The main reasons why most are built this way seems to be consumer expectations of domestic acceptability and manufacturing convenience.
The following extract is taken from diyaudioandvideo.com
What is the best shape for the box?
Internal reflections in the box combined with the vibration of the box itself can cause spikes in the frequency response of the system.
Different box shapes have different effects, with perfect cubes being the worst and spherical or egg shaped boxes being the best.
Although spheres have advantages, it is very difficult to create a spherical speaker box that is as strong as a typical rectangular box.
One good example of a spherical speaker is the Gallo Nucleus Solo above on the left which is made of rolled steel or brass.
An even better design is the sphere/tube concept by B&W shown on the right.
This design gets the benefits of a spherical design, but also adds a tapered tube at the back to eliminate all internal resonances.
Shape VariationCube +-5.0 db
Rectangle +-3.0 db
Cylinder +-2.0 db
Beveled Cube +-1.5 db
Beveled Rectangle +-1.5 db
Sphere +-0.5 db
https://www.diyaudioandvideo.com/Guide/BuildSpeaker/