What about cities? Do they defend themselves with stationary cannons? They wouldn't need to move very far from the smithery. No army can lug around giant cannons with them, but cities don't move, and if Polytopians can transport them onto ships, they can transport them a short distance away to the edges of the city.
So, we've established that the Ragoo cannons are pretty much a danger to everyone, even the crew.
Thats why, despite using gunpowder, they're just as effective as Catapults. They're large, clunky, hard to reposition, cumbersome, loud, and always at risk of exploding, and thus, aren't much better than a catapult.
City defenses don't really have them because they'd be too big for city walls, most likely, and there's too big of risk of collateral damage.