Because projects are seldom as simple as that, potentially having lots of other kinds of dependencies (a directory structure, system libraries/command, symlinks

Because projects are seldom as simple as that, potentially having lots of other kinds of dependencies (a directory structure, system libraries/command, symlinks, other services running, etc). And let's be honest, virtualenvs and all the various ways they're managed and updated and such aren't really bulletproof either. It's really, really refreshing how much "yeah I managed to break my dev environment" or "I followed the wiki for how to start developing your project but it 'didn't work'" can be avoided if it's just "run docker(-compose)?". And this is especially true for junior developers, who are probably fresh out of college and won't be familiar with lots of the tooling that exists in the world. Not that docker is a simple tool, but it can hide so much complexity that it is easier to just show people how to docker run and docker build and such.