[Twisell]

But a positiv counter argument would be that github/gitlab also does the sharing part of the jobs and you (hopefully) pay them to handle all the security aspect of that public sharing.

Opening a port to your local instance of git implies more thinking and security consideration. Of course you could host a mirror on a cloud instance, but then it's saas anyway and you have even more work to do before ever starting coding.

So yeah github/gitlab does the heavy lifting for you, but for small organisations that might be cheaper overall than to pay somebody mastering all required stack to implement and maintain your own instance(s). For big company no question that an in house git team is probably more efficient.