Yet children have cell phones and use the internet right? Nothing (as far as I know) will stop a child from walking into a walmart and buying a pre-paid phone or a TV or a computer but all of those things will force you agree to their terms.

How enforceable all of that is legally would be up to the courts, but that's true for social media sites too. If a 15 year old signs up for a social media site they're forced to agree to terms that might not hold up in a courtroom. None of that protects the child from having their data harvested, sold, and used against them for the rest of their lives.

If we agree to ban kids from using social media on the basis of them not being able to legally enter into a contract, we'll have to ban kids from a whole lot more that social media since so many things in our lives require one. Even things explicitly made for children.

I'd be better to outlaw the data collection and exploitation than to tell kids they can't use cell phones or computers or social media and televisions. No need to limit that to just kids either.