What a difference between unitypackage and using git to share asset

A unity package is a way to share some assets of your project so they can be added and used in other projects.

Git is a version control software and it's intended to be used for the development of one project. It keeps registry of every version of each assets of your project.

They're not similar things you should be comparing and choosing between them, they're used for really different things.

As for your question, one obvious thing you can't do with git is sharing your assets in the Asset Store, you have to upload a unity package there. But it's not because git is worse or something, it's because it doesn't make sense.