You're correct. They secrete layers of calcium to grow it, but their glands can't reach the old parts of the shell, so it doesn't get repaired or maintained.
I accidently got a population of snails in one aquarium, and they really added beauty and specks of interesting life on both terrain and glass. Their shells where tiny but pretty, and I can really see how nice larger ones would be in a terrarium.
I'm fortunate enough that I can just go outside in my garden in the summer to see them, but if I lived in a city that would definitely be a nice habitat to keep