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.