Because a user sitting in the United States could be served copyrighted content by GitLab. It would likely even come from one of GitLab's servers in the US. In that hypothetical instance, GitLab is in clear violation under US law.
In theory GitLab could decide to ignore the DMCA, as you suggest, but that would mean removing all US servers, firing all US staff and cancelling all contracts with US customers (including those that GitLab has with the US government itself). Even in that instance, you would just move the copyright lawsuit to Dutch courts.
In theory GitLab could decide to ignore the DMCA, as you suggest, but that would mean removing all US servers, firing all US staff and cancelling all contracts with US customers (including those that GitLab has with the US government itself). Even in that instance, you would just move the copyright lawsuit to Dutch courts.