FreeCAD uses OpenCASCADE, because it's the only free option. You will notice that almost no other 3D Program, paid or free, uses it. OpenCASCADE is, compared to Parasolid or ACIS; buggy, broken, incomplete, unergonomic, and obsolete. There is almost no commercial software that uses it - they all find it better to license Parasolid or ACIS despite the heavy royalties those packages require.
This puts FreeCAD in kind of a chicken-or-egg situation. The geometry kernel they use is considered unanimously inferior to the licensed kernels for almost every task, to the point that even people building new CAD programs (like the recent Plasticity beta) don't even consider it as an option. The 3D packages that license the kernels think that rebuilding ~35 years of work with extremely complex math equations requiring dozens, or even hundreds, of PhD-level math professors is too much work to justify not paying a license fee. To them, it's like saying we should rebuild Windows, but it's PhD-level math instead of APIs.