While we're on the topic... the name requirements.txt is so generic and undescriptive. It breaks the principle of least astonishment. Something like packages.pip would make more sense.
What does that even mean?
A .txt file is meant for human readable sentences, yet this file is meant to be consumed by pip. You can only "customize" it working within the confines of pip freeze. If you were new to programming, would you suspect that requirements.txt was somehow linked to Python? Probably not.
Why can't ruby or JavaScript or any language make use of requirements.txt? They could, and make the same claim you just made, which would break pip install -r.
For a comparision, NPM has a packages.json which is hand-editable, and lots of people do customize it by hand to get their NPM scripts running, but:
a) If you're new to programming, you might be confused by which language it belongs to just like a requirements.txt
b) if you peek into it, you may not think it's generated, consumed, or editted by machines because JSON format is so readable
My point is that the .txt extension doesn't give any hints to its use or that it's breakable, and that the name "requirements" doesn't describe the intended domain of the file (Python only). Even naming it "python_requirements.txt" would be an improvement.