`pip install` should only be for the default way that projects define dependencies, where the default is the way. Were we to have both pipfile, and requirements.txt, and maybe a third, and a fourth option, I don't think `pip install` should work.
I also don't think it's ever a good idea to have multiple options for something like that.
Especially for newcomers, an implicit "use Pipfile if exists, else requirements.txt" is just unnecessary magic.