Topic on Talk:Environment

The benefit of individual pages is that you can use the infobox template to make referable variables. For example, you can use {{Q|Rough stone|Move Speed Factor}} to automatically output 0.87, and have that number be updated whenever the page is. Similarly those properties can be used in math, or called as part of #asks (e.g. to show all terrains and flooring types with Move Speeds under 100%). A combination of these are how many templates around the wiki work.

So far it hasn't been necessary to do that for terrain pages, though if any compelling use case exists they could be created.

As for the specific examples, rough and roughhewn stone had at least some content and I wasn't go to go out of my way to delete the page. Shallow water was basically a single line, half of which suggested going to watch a youtube guide iirc.