I've made sure that pip-licenses is installed in the same environment that I'm running the command in. It is the only package missing in the output despite using the --with-system flag – prettytable and wcwidth are included.
I believe this happens since version 4.0.0.

$ pip --version
pip 22.3.1 from /home/***/.virtualenvs/***/lib/python3.10/site-packages/pip (python 3.10)

$ pip freeze | grep pip-licenses
pip-licenses==4.0.0

$ which pip-licenses
/home/***/.virtualenvs/***/bin/pip-licenses

$ pip-licenses --with-system | grep pip-licenses

(The final command produces no output)