There is no need to use CSG in the game engine, as everything is real-time and dynamic.
That's nonsense. Like saying there is no need to use physics, or lights in the game engine, as everything is real-time and dynamic.Whether there is a reason for using CSG in a game would depend primarily on what game we're talking about.
Some older games like Quake 1 used CSG maps, as they didn't have real-time engines by that time.
What do you mean by a "real-time" engine, and why isn't Quake 1 one of them? Besides Quake 1 did not use CSG maps. Games like Red Faction appeaerntly did use CSG (or some similar technology), at least for the parts of the maps you could destroy.