Again, your error is treating it as a transport.
A steamwagon will let you get a battery of cannons from your city to an enemy city in a small fraction of the time that conventional wagons or gun carriages could. And then let the cannons fire from an armoured bunker rather than an open field.
They are tanks, not trucks.
While it is undoubtedly true that any given enemy has the potential to deal with something like a steamwagon then same can be said for everything else. Warriors, Rangers, Slayers, cannons, etc all have their weaknesses and counters.
Whether adding landships to your order of battle is worth the cost is an entirely different calculation than whether they should be used for carrying stuff between allied destinations.