Sharing buttons means to share the content of particular page, post or project. So, when you will enable sharing buttons on particular destinations, they will appear under the post/page/project content automatically by Jetpack.
Home Page is a placeholder for different projects. So, when you will enable sharing buttons for projects, they will appear under each project to share. In a more simplified way, projects on the home page are not the content of homepage, these are actually projects content which is appearing on home page.
This is not option by Jetpack.