Because let's say you have a website like Reddit with a mixture of 18+ and 13+ stuff. Something that's 18+ but not explicit gets really popular. Pornhub will now livestream congressional debates, someone posts a link to this, and it gets fifty zillion upvotes as people can't resist commenting "wtf". Now you have a dilemma: do you show it on the front page or not? If you show this on the front page and it's 18+, it'll lock children out of the front page, which for many of them means they're locked out of the entire site because that's the way they know to access the site. But if you don't, then the law is forcing you to censor your front page for everyone, even for adults.
The solution is obvious: you show it on the front page if the user is 18+. However, your proposal deliberately forbids this and says the front page must be the same for everyone. Which leaves the other two bad options.
The solution is obvious: you show it on the front page if the user is 18+. However, your proposal deliberately forbids this and says the front page must be the same for everyone. Which leaves the other two bad options.