not including the python bindings in the Spack package is intentional. The proper distribution of the Python bindings is still an open todo (see for example https://github.com/precice/precice/issues/436, https://github.com/precice/precice/issues/512, https://github.com/precice/precice/issues/381).
As a workaround for your problem, I suggest that you clone preCICE from git (make sure you use the same version that you are using with Spack). This will give you everything you need for installing the python bindings using pip. Now install the python bindings. This is ugly, but currently necessary.
Our future plan is to put the python bindings into an independent repository and get rid of everything that is related to the python bindings within precice/precice, soon. If you want, you can also open an issue for your problem or add a comment in one of the already existing ones.