The real problem is that BREP CAD kernels are hard. A few of proprietary kernels dominate the scene: Parasolid powers NX, SolidWorks, Fusion, and Onshape, while ACIS (owned by Dassault) is used by Inventor and BricsCAD. Catia uses Dassault's own CGM kernel. The open-source world relies mostly on OpenCASCADE, which is unfortunately a lot less capable than any of these.

Fillets and chamfers are a good example. They seem simple but are geometrically non-trivial, and OCC will fail on cases that Parasolid handles without complaint. You can push either kernel to its limits if you try hard enough, but OCC hits that ceiling much sooner. So any CAD tool built on top of it inherits that ceiling too.