In the 18th-19th centuries western battlefield, why didn't they make large steel barriers on wheels to give them cover as they approached the enemy lines?

Is there a reason why steel curtains wrapping a wooden wall weren’t used as mobile shields to give advancing armies cover from musket fire? I understand that they’re not going to do much against a cannonball, but the tactics of just charging unprotected into a hail of enemy gunfire seems totally asinine.

Obviously there were smart people back then and what I’m describing seems like a rather obvious tactic that likely would have been used if it were effective. This implies that there would be major problems with such a tactic that I’m overlooking.