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.

If you want something based on B-Rep, look at projects that use opencascade under the hood, as that is one of the only B-Rep CAD kernels available which is free and open source. Some examples would be CADQuery, CascadeStudio, or RepliCAD.