There is an important difference in the metaphor. Physical spaces don't have a user-agent. Therefore the space itself has to exclude minors because there is not

There is an important difference in the metaphor. Physical spaces don't have a user-agent. Therefore the space itself has to exclude minors because there is nothing else to do it. The internet does have user agents, and for children the user agents are controlled by adults (and if the child has the cooperation of an adult, you already can't enforce age verification). So now you don't need anyone to prove their age in a way that has privacy implications and chilling effects because you can have the child's user agent notify the site that the user isn't an adult, rather than needing each adult having to prove who is. Which doesn't require any form of identification because it's just a flag the adult sets on the child's device. Therefore anything that does require identification is unnecessary and malicious.