Because of drunk driving. If that was not an issue, we certainly wouldn't have a drinking age of 21 in the US. Several states still allow your family to serve their children alcohol, which is closer to the example of letting your kid use the internet.
The restriction was because of concerns around minors being too young and naïve to fully understand the full implications of their actions. Which is why consumption is/was allowed at home on the basis that responsible adults will help children to learn to handle alcohol sensibly while stopping them doing anything too stupid ... at least that's the theory anyway.
Similar rules apply to driving, voting, smoking etc in most places. And as it is possible to live a perfectly meaningful life without being bombarded by TikTok, Instagram, Facebook, YouTube etc, it's really hard to understand why similar should not apply to Social Media.