It's easy to convert because CSG is a small subset of what BREP can do.

It's analogous to "all squares are rectangles, but not all rectangles are squares" (squares=CSG, rectangles=BREP)

CSG by itself isn't suitable for most CAD use-cases.

How can I convert openSCAD models to true STEP files? As in no meshes. I Literally have wanted that for years.