It's a fundamental misunderstanding of 18th and 19th century infantry tactics to say that they fought that way out of a sense of decorum and that no one had the idea of using concealment or cover.

They fought in massed formations because it actually was the most effective way of fighting with the technology of the time. Infantry troops that strayed too far from their formations were extremely vulnerable to attack by more mobile cavalry troops. It was only in massed formations that they were able to effectively repel cavalry attacks.