Administrator should not automatically be able to merge everything
I understand the rational "The Admin can give themselves any permission they want, so why not activate everything for them beforehand"? This reduces the false sense of security one could get if they see an admin not being able to do a certain thing (but forgetting that they could give the permission themselves).
However, this blurs the different roles of a user in different projects. If the person responsible to manage the gitlab instance (==admin) is also part of the development team (and in most non-huge enterprises this will be the case), this does not necessarily mean that they are maintainers for every project.
But right now, they always see the "merge" button and might forget in some cases that they shouldn't merge on their own but leave it to a "true" maintainer.
I think not having the permission by default would separate the areas of responsibility better. As nearly all of the Gitlab permission issues, this feature should maybe be configurable