So for Linux, generally you are installing packages from your distro's repo so they are signed by the repo itself. I would have assumed that it would be the same on Mac with brew/macports/etc signing the code, but from what you are saying I guess not, I don't see why.

On windows you just need a certificate from a known authority. This will still probably cost you money but you have a lot more options at different price levels. Also that certificate is a widely useful thing rather than an apple dev account which is only useful in the apple walled garden.