Python absolutely can run scripts in installation. Before pyproject.toml, arbitrary scripts were the only way to install a package. It's the reason PyPi.org doesn't show a dependency graph, as dependencies are declared in the Turing-complete setup.py.

Wrong. Wheels were available long before pyproject.toml, and you could instruct pip to only install from wheels. setup.py was needed to build the wheels, but the build step wasn’t a necessary part of installation and could be disabled. In that sense its role is similar to that of pre-publish build step of npm packages, unless wheels aren’t available.