Baking in a rectangular shape allows you to make a space efficient bread that you can easily stack and transport. Also, it is very predictable, can fit neatly into your toaster, and can be cut in triangles.

Making bread on a flat surface allows you to minimize costs of entry (not only don’t you need the forms which are relatively cheap, you can go with simpler/cheaper ovens), and this kind of bread has a more pronounced crust, which many people like.

Also, rectangular bread is harder to leaven for a long period of time as it comes with numerous technological complications down the production line. This affects the aroma composition, making rectangular bread less attractive for those who want the traditional “bread” taste.

Baguette, as I already mentioned, has a unique crust and crumb texture defined by the shape and baking conditions. Many people like it that way.