It’s because certain jurisdictions require companies to not allow access to potential 18+ content without getting consent and the user saying they are over 18. If there isn’t an age selection then in their T&Cs it’ll likely have something saying that creating an account confirms you are over 18.
You could just use a popup that says “Are you over 18 and willing to see NFSW content?” like most NSFW website do, but I guess that doenst generate enough money unlike forcing accounts does.
That’s not always the case because signing up and accepting a T&C makes it more binding of the user to say they are who they say they are. If they aren’t, then it’s close to fraud. Having just a “are you over” screen is not enough, since the user can easily lie and the company can get done for not having enough measures in place to stop that from happening.
I know in the end it’s all futile anyway because of many reasons. But for companies it’s checking off a box that gets a governing body off their back and making sure they have the right certifications to do what they need/want to do.
Since when does a reddit account require me to submit ID proving I am indeed 18+? I literally see no difference in reliability between requiring an account where one claims to be 18+ and a banner where one has to claim to be 18+. In both scenario’s you are relying 100% on user input.
