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?

it seems nobody wanted to attempt that until motorized armor (the tank) was created during World War I.

Random thoughts picturing myself in the situation:
—Pushing a steel vehicle would be slow, especially over rough terrain
—If the terrain is smooth and flat, then the other side could move fast enough to swarm you
—You could get flanked by fast-moving cavalry
—If you were protected on all sides, you were trapped inside the thing if it was stopped
—A few turtles (for lack of a better name) would not have firepower to do anything once they got forward
— A long line of turtles couldn’t advance as a line except in perfectly clear terrain
—Perfectly clear terrain is not where armies set up defensive lines

Where this concept actually did happen of course was on the water. Ironclads I guess have the mobility and really thick armor to make it work.

Try pushing around a large steel barrier on wheels. Now imagine coordinating thousands of men with hundreds of those things. The canon were hard enough to drag around, using horses. It would be way too much to have soldiers carry around. Even in cases and places where it were possible, it would slow down your army and fatigue it. A slow tired army can be out-maneuvered.

Not to mention the metalworking required to create steel walls.

Another example where something like this might be used is fortification sieges. Build a fort to face the fort you’re besieging. Although those tended to be more theoretical and avoided in practice in that period.

In both the 18th and 19th century, cannon fodder was more plentiful and far less costly than steel. More telling is the fact that the tactics pursued in warfare ALWAYS lag considerably behind advances in weaponry, and in this it is ALWAYS the infantry which suffers the brutally lopsided consequences.

@stanleybmanly I was thinking something similar. Soldiers’ lives were expendable; you just needed to have plenty to spend.

I would blame that on the people in charge being traditionalists.
I mean, consider how long after the widespread use of guns people still used tightly packed formations inherited from the age of melee combat.

Build a fort to face the fort you’re besieging

That was done in the past. I think cannon ended the practice. Gunpowder allows shooting at the castle from a safer distance.

‘Wikipedia – A siege tower or breaching tower (or in the Middle Ages, a belfry is a specialized siege engine, constructed to protect assailants and ladders while approaching the defensive walls of a fortification. The tower was often rectangular with four wheels with its height roughly equal to that of the wall or sometimes higher to allow archers to stand on top of the tower and shoot arrows into the fortification.’

The sad truth is that nations all too frequently make the mistake of preparing to fight the previous war.

That is why WW1 with its trench warfare was such a shit show. They just were not prepared to deal with this new paradigm of war.

No battlefield is level, much less free of obstructions. If these mobile defenses get pushed out there, they might have to be pulled back. And what to do with the wounded that occur behind these heavy mobile forts?
Back in those days all they had was cast iron, not steel, which would shatter when struck with a cannonball. And, of course, there is the mud…
The logistics would have been daunting.

A shield wall such as the ancient armies used in phalanx formation might have been a more practical equivalent, but soldiers stopped carrying shields a while back.

Grapeshot*, when fired from a cannon, would inevitably find any flaws in a shield. Like exposed feet. (*Not to be mistaken with case shot/canister shot, which would do the same thing only at closer range.)