Sourdough bread can indeed be baked in a pan to make it square or rectangular, similar to how traditional sandwich loaves are made. However, the reason sourdough is often baked in a more free-form shape (like a boule or batard) or in a Dutch oven rather than in a loaf pan includes several factors:

Crust Development: Sourdough enthusiasts often prefer a thick, crispy crust, which is better achieved in a free-form shape with the direct heat of an oven or within a preheated Dutch oven. A loaf pan might limit the crust development on the sides and bottom.

Artisanal Tradition: Sourdough baking is deeply rooted in artisanal traditions where the rustic appearance and unique shape of each loaf are valued. Free-form loaves embody this artisanal quality.

Oven Spring: Sourdough, being a naturally leavened bread, benefits from the high heat and space to expand freely, which is best achieved outside of the confines of a pan. This allows for better oven spring, the rapid expansion of the dough during the early stages of baking.

Heat Distribution: The method of baking directly on a stone or in a preheated Dutch oven provides even heat distribution, which can be more challenging to achieve in a pan, particularly for the thicker, denser dough characteristic of many sourdough recipes.

Texture and Shape Preference: Many bakers and consumers prefer the texture and crust-to-crumb ratio of round or oval sourdough loaves. The aesthetic and texture of these shapes are part of the sourdough bread’s appeal.

That said, sourdough bread can absolutely be baked in a loaf pan if a square or rectangular loaf is desired, especially for sandwiches. This method is less traditional but entirely feasible and can result in delicious bread with a softer crust and uniform shape. Bakers adjusting sourdough recipes for loaf pans may need to tweak hydration levels and baking times to account for the different baking dynamics.