Because then: whose computer is it? If it's my computer, it does my bidding. If a child is currently on the computer, should the computer decide and refuse to work as in "I'm sorry Dave, I'm afraid I can't do that" where the operator (Dave) is deemed a threat to its objective? In this case, the computer's programmed goal would be preventing any minor from seeing content with this tag. It has to decide if you're a minor whenever content is marked as such. If you can't override its programming, it's your computer no longer
Or is it because, as others have mentioned, it’s not really about protecting minors from adult content?