The reality is build123d is a fairly thin layer over the OCCT kernel with some pythonic affordances. I'm not sure OCCT has a robust constraint solver, so there is little development there. FreeCAD on the other hand (besides being significantly more mature) is also build on OCCT, but also does a lot on top of the kernel to make it more featured and stable.
Parent was complaining about the lack of a built in sketch constraint solver in build123d, which is table stake feature of any "real" GUI CAD suite. build123d has some constraint options and you could probably use solvespace with it. Of course you can roll your own.
