You're correct in that keeping the packages in the dockerfile instead of the requirements file might save some time during development. However, I do prefer the requirements file in that it's cleared and easier to understand, both for other developers and oneself.
Requirements files also make it easier to use different files for different environments (thanks to their extensibility) while that could only be approached in docker using different images that depend on one another (or just entirely different dockerfiles that all mention the same packages, which breaks DRY), and might eventually require the introduction of tools such as docker-compose.
There's also the issue of portability to keep in mind. If the team is already using python, it and pip are a given for all the project's developers, while docker is more optional.