which venv does really well, what am I missing.

in the "worst" case you can always do a docker

venv only works if you already have the required version of python installed and have it active as the python for your shell (or use a launcher to which you specify the python to use in the command line), since venv just creates a virtual environment based on the version of python it is run with.

So you actually need something that can read a specification of the required python version and use the correct one from the available options (maybe even reaching out and getting it if it isn't already locally available, though I don5 remeber if any of the existing python build tools will do this; docker obviously will, but its not really a python build tool), across different OS flavors (maybe not that last bit, depending on the use case, especially for internal development), and with minimal overhead for the build tool itself.