Because even if we go with the entry-point-style Python
hooks, the build frontends like pip will still want to spawn a child
to do the actual calls -- this is important for isolating pip from the
build backend and the build backend from pip, it's important because
the build backend needs to execute in a different environment than pip
itself, etc.
[...]
Concretely, the way I imagine this would work is that pip would set up
the build environment, and then it would run

  build-environment/bin/python path/to/pip-worker-script.py <args>

fwiw, such a worker is what I was describing in an earlier thread with Robert last work

https://mail.python.org/pipermail/distutils-sig/2015-October/027443.html

although I wasn't arguing for it in that context, but rather just using it to be clear that a python api approach could still be used with build environment isolation