You can just read the source? mixxx-deps come from a build process sourced from:
https://github.com/mixxxdj/vcpkg
The same binary-backed build process is present on Windows, too, presumably to keep people from needing to go through dependency hell to contribute to the project.
That is exactly the reason. Also, some libraries we are using are not on homebrew (and we cannot easily pin a specific version).
Another reason that we'd like to reduce the maintenance burden by using a similar setup for windows and macOS deps. Our dev team is small, and almost all of us use Linux. For macOS in particular we don't have anyone on the core team that uses it as their daily driver, which may also play a role when talking about the DX in macOS.
Mixxx is entirely community driven: There is no company behind it, everything is done by unpaid volunteers.