Other third party libraries are vendored in to the official Python release, such as sqlite and Tcl/Tk, with standard library wrappers
So, it wouldn’t be impossible to do the same for pip, but there would need to be willingness from Python core devs and pip maintainers, and that seems unlikely.
Yes, whether pip provides a Python library API is a question for the pip maintainers (such as myself and @pf_moore), I’m personally of the opinion that providing one would be ideal, but given the history and maintainer resources it seems unlikely to ever happen.
Previous discussions can be found on the issue tracker and even here on DPO: