There are third-party tags out there compatible with both Google and Apple's network that is roughly the same size and use the same battery, yet have a giant lanyard opening in the design to fit anything.

Apple could trivially have fit a usable hole if they wanted to. They just don't want to because they get to sell accessories with that now. Also, looking cleaner on its own helps sell even if that is an entirely useless quality for a tag tha tneeds to go into a bloody case.